Thank you to Tees. Definitions are underlined in the clues.
Across
1. Direction of take-off in this plane! (8)
VERTICAL : Cryptic defn: Reference to a VTOL (vertical take-off and landing) aircraft/plane which takes off in the vertical, as distinct from the horizontal, plane.
6. Good man with something to cover flasher at disco? (6)
STROBE : ST(abbrev. for “saint”, a good/holy or virtuous man) plus(with) ROBE(something/a long loose garment to cover).
9. Potassium density fluctuates with force and atomic explosive (5,2,8)
STICK OF DYNAMITE : Anagram of(… fluctuates …) [K(symbol for the chemical element, potassium) + DENSITY plus(with) F(abbrev. for “force” in physics) plus(and) ATOMIC].
10. Difficult problem for Ravel (4)
KNOT : Double defn: 2nd: A tangled mass.
11. Work experience aboard vessel protecting marine species (10)
INTERNSHIP : [IN(aboard/on) SHIP(an ocean-going vessel)] containing(protecting) TERN(a species of seabirds/a marine species).
12. Theatre skill doubled energy in comic dialogue (8)
REPARTEE : REP(abbrev. for “repertory”/a theatre staging various plays, operas, or ballets by a company) + ART(skill/the competence to do a particular thing) + E,E(2x/double the abbrev. for “energy” in physics).
14. Solemn promise to refrain from drinking toast (6)
PLEDGE : Double defn: 1st: …, say, alcohol; and 2nd: To express support or goodwill by drinking to a person or cause/a toast.
15. Second customer calling for house cat (6)
MOUSER : MO(short for “moment”/a short period of time, like a second) + USER(a customer/one who uses a product or service).
Defn: Calling/a job for a house cat, viz. to catch mice.
16. Smoke hid rocky Arab territory (8)
SHEIKDOM : Anagram of(… rocky) SMOKE HID.
17. Revolutionary and monster becoming accessories (10)
ADORNMENTS : Anagram of(Revolutionary) AND MONSTER.
19. Horse to follow flawlessly finding way through (4)
PATH : H(abbrev. for “heroin”, aka “horse”) placed after(to follow) PAT(flawlessly/exactly, as in “he gave the answers pat”).
20. People opposed to the enemy are in a rush (4,7,4)
RACE AGAINST TIME : RACE(a group of people with common features or shared ancestry) + AGAINST(opposed to) + TIME(the enemy, as in the quote “Time is my greatest enemy”, attributed to Eva Peron).
21. Payment letter demands material about conservationists (6)
RENTAL : REAL(material/tangible) containing(about) NT(abbrev. for the National Trust, entrusted with conserving places of historic interest or natural beauty).
Defn: …, a letter being one who lets out property.
22. Performer takes action to return in Hamlet? (8)
DANSEUSE : Reversal of(… to return) SUES(takes a court action against someone) contained in(in) DANE(an example of which/? is Hamlet).
Defn: A female ballet dancer.
Down
2. Understanding directions about shelter put up in centre (7,8)
ENTENTE CORDIALE : E,NE(abbrev. for the directions “east” and “north-east”, respectively) containing(about) TENT(a portable cloth shelter) + [reversal of(… up, in a down clue) LAID(put on/placed on) contained in(in) CORE(the centre/heart of something)].
3. Songbird catching a cold becomes silent (5)
TACIT : TIT(a songbird) containing(catching) [A + C(abbrev. for “cold”)].
4. Boy singer with his rector for excitement (9)
CHORISTER : Anagram of(… for excitement) HIS RECTOR.
Defn: … in a choir.
5. Girl guzzling due to drain coffee cups (7)
LADETTE : “due” minus its middle letter(to drain) contained in(… cups) LATTE(coffee drink made with espresso and steamed milk).
Defn: The female equivalent of the “new lad”, given to drinking heavily/guzzling alcohol, among other things.
6. Rocket incorporating new submarine detection system (5)
SONAR : SOAR(to rocket/to fly fast and high) containing(incorporating) N(abbrev. for “new”).
7. Cut wood supporting seats (4,5)
RUMP STEAK : TEAK(a hard durable wood) placed below(supporting, in a down clue) RUMPS(seats/the buttocks).
Defn: … of meat.
8. Substance got him dancing in swimwear (7,8)
BATHING COSTUMES : Anagram of(… dancing) SUBSTANCE GOT HIM.
Then: 
… and now: 
13. Taking it all in having gone round planet? (9)
ABSORBENT : ABSENT(having gone/no longer here) containing(round) ORB(literary term for a celestial body like a planet, roughly in the shape of an orb).
14. Mixed responses but continues forward (7,2)
PRESSES ON : Anagram of(Mixed) RESPONSES.
16. Daily porridge needed for African flier (7)
SUNBIRD : SUN(the daily British newspaper) + BIRD(slang, like “porridge”, for a prison sentence).
18. Be consumed in chimney erected for tycoon (5)
MOGUL : Reversal of(… erected, in a down clue) [GO(be/to happen, as in “Let it be” and “Let me be there”) contained in(in) LUM(a chimney, from Scottish)].
19. Film company sorry having removed jerk (5)
PATHE : “pathetic”(sorry/miserably inadequate) minus(having removed) “tic”(a jerk/an involuntary contraction of a muscle).
Some wonderful long anagrams.
19A I took the “h” to stand for – horse, in racing usage everywhere a horse being an entire (ungelded) male of four-years-old and over.
11A is TERN in IN SHIP
Thanks to Tees for the entertainment and scchua for the blog.
Thanks gwep. Oversight in 11a corrected.
A lot of fun and a bit of relief after the struggle to finish off yesterday’s Guardian. Lots to like although I didn’t much care for 1a. I took “h” for “heroin” but I’m sure “horse” in the non-drug meaning is also fine. Thanks both.
Re 20 across, I’m pretty sure time being an enemy predates Eva Peron. The phrase ‘how goes the enemy’ meaning ‘what is the time?’ Is certainly of nineteenth century origin, and there are probably much earlier examples.
The usual very high standard from Tees, quite something for someone who has become quite a prolific setter.
Thanks S & B
Elegant clueing, as always with Tees, and I thought 9A was splendid. Despite my previous comments about SE corners, it was the NW that held me up today.
My favourite clues were 7D and 6A but the whole puzzle was nicely clued. It was an enjoyable steady solve.
Thanks to Tees and scchua.
jamc, to clarify. I didn’t mean that the time/enemy sense originated with Eva Peron. As with other instances, I was giving an example of usage, in this case a quote attributed to Eva Peron.
Most enjoyable and solved without any problems. Favourites were STICK OF DYNAMITE and MOUSER. Thanks, Tees and scchua.
I have observed before, but in another place, the irony that Time is referred to as both the enemy and the healer.
Another really neat puzzle from Tees and, as Tatrasman says, elegant as ever. I just love the surfaces this setter achieves: smooth, clever, often misdirecting and often beautifully relevant to the solution. On this occasion, STROBE, REPARTEE, SHEIKDOM, DANSEUSE, CHORISTER, TACIT, RUMP STEAK and BATHING COSTUMES all earned big ticks. I note but am not bothered by PATH and PATHE sharing an initial letter; both were so differently clued, I see nothing to complain about (and I’m afraid I’m one who immediately thought heroin rather than quadruped to arrive at the ‘h’.) [How different might have been Thomas Gradgrind’s preferred definition of horse had Dickens been writing about gritty urban life in the 21st Century …]
Thanks Tees and scchua for the blog (especially identifying the anagram fodder for STICK OF DYNAMITE. The solution had to be that and I confess to not piecing together all the constituent elements)
Scchua @7, I see what you mean. Thank you.