I found this to be a bit of a mixed bag, some very easy entries and some, such as 18ac, that took a little while to sort out the parsing.
There are quite a few forenames in the clues (Miles, Penny, Henry, Victoria, Georgia, Frank, Rex, Peter & Mark) but I cannot see anything that connects them so I suspect in most cases it is just false capitalisation in an attempt to deceive the solver.
Across
1 Study by country’s leading economist (6)
PERUSE – PERU’S (country’s) E[conomist] (leading economist)
4 Cheap containers (6)
TINPOT – TIN POT (containers)
8 Insect endlessly eating into house plant (7)
VANILLA – AN[t] (insect endlessly) in (eating into) VILLA (house)
9 Works as party secretary at first (7)
LABOURS – LABOUR (party) S[ecretary] (secretary at first)
11 Unsettled by endless touring Sheeran retired (10)
INDEFINITE – INFINITE (endless) around (touring) ED (Sheeran) reversed (retired)
12 One having an edge over Miles in test (4)
EXAM – AXE (one having an edge) reversed (over) M (miles)
13 Safe-breaking expert not unknown (5)
PETER – an anagram (breaking) of E[x]PERT (expert not unknown)
14 Cooperate with the French by turning over everything on Penny (4,4)
PLAY BALL – LA (the French) BY reversed (turning over) ALL (everything) after (on) P (penny)
16 Persuade city fellow to stop working? Just the opposite (8)
CONVINCE – ON (working) in (to stop) C (city) VINCE (fellow)
18 Frequently getting to start with us for a bit of practice (5)
OFTEN – OpEN (to start) with FT (us) replacing (for) p (a bit of practice)
20 Pants / of inferior quality (4)
POOR – double (or not so) def.
21 Talked about managing change without Henry or Victoria for instance (6,4)
SPONGE CAKE – SPOKE (talked) around (about) an anagram (managing) of C[h]ANGE (change without Henry)
23 Pester daughter to get out of nearby Georgia (7)
BESIEGE – BESI[d]E (daughter to get out of nearby) GE (Georgia)
24 Lie when in charge after fellow gets into trouble (7)
FICTION – F (fellow) IC (in charge) plus an anagram (trouble) of INTO
25 City father not initially surprised by hint of nepotism (6)
TEHRAN – an anagram (surprised) of [f]ATHER (father not initially) N[epotism] (hint of nepotism)
26 Manages to pass without honours eventually (4,2)
GETS BY – GET BY (to pass) around (without) [honour]S (honours eventually)
Down
1 Frank’s a tool they say (5)
PLAIN – sounds like (they say) ‘plane’ (a tool)
2 Try wearing torn clothing (7)
RAIMENT – AIM (try) in (wearing) RENT (torn)
3 Continue doggedly with idols Nero replaced (7,2)
SOLDIER ON – an anagram (replaced) of IDOLS NERO
5 Furious with deserter found in the middle of Amiens (5)
IRATE – RAT (deserter) in (found in) [am]IE[ns] (middle of Amiens)
6 Saw priest in preference to bishop (7)
PROVERB – PR (priest) OVER (in preference to) B (bishop)
7 Material of value mostly returned to poet (9)
TARPAULIN – RAT[e] (value mostly) reversed (returned) PAULIN (poet) – Tom Paulin
10 Fool about after lips met excitedly (9)
SIMPLETON – ON (about) after an anagram (excitedly) of LIPS MET
13 Advocate removing power from adversary in support of prince (9)
PROPONENT – PR (prince) OP[p]ONENT (removing power from adversary)
15 Juxtaposed with a theatrical device involving trousers for the most part (9)
ALONGSIDE – ASIDE (a theatrical device) around (involving) LONG[s] (trousers for the most part)
17 Rex checking Peter out in Japan (7)
VARNISH – R (Rex) in (checking) VANISH (peter out)
19 Implied without being stated by American historian (7)
TACITUS – TACIT (implied without being stated) US (American)
21 Mark ignoring Iseult’s last letter (5)
SIGMA – S[t]IGMA (mark ignoring [iseul]t)
22 Strange piece of computer software by the sound of it (5)
KOOKY – sounds like (by the sound of it) ‘cookie’ (piece of computer software)
As you say – a mixed bag. Lots to like, but quite a lot I didn’t like personally. Not keen on c for city in 16a. 20a seems to be essentially the same definition twice – a ‘poor’ clue methinks. Never met the poet Paulin, but that’s not a criticism. IMHO, kooky doesn’t sound like cookie, but I guess it might to some. Thanks to Peto and Gaufrid.
Thanks Peto and Gaufrid
Wasn’t able to complete this for reasonsof time as muc as anythng else.
Defeated by VARNISH, as I don’t think VANISH and PETER OUT are synonymous – the former is, to me, much more abrupt than the latter.
I knew that when I had Peter for Safe (13 Ac) and Proverb for Saw (6 d) – after some deliberation – that I was not going to finish this and I duly didn’t.
Thanks Peto and PeeDee
This was a toughie that I didn’t get to until Friday and it spilled over into late Saturday for the grid to be filled in.
Still, I could not parse a number of them fully:
OFTEN (my last one in and even thinking of the FT for ‘us’ could not make the last leap of faith to replace P (a ‘bit of practice’ – really?)),
POOR (didn’t know the British slang meaning of ‘pants’ – so lucky that it was a double definition of sorts),
SPONGE CAKE (hadn’t heard of the Victoria sponge – so didn’t know why she was hanging around),
ALONGSIDE (didn’t resolve that there was an S that needed to be taken from the long trousers to allow ASIDE not just AIDE) and
GETS BY (where I had it as a weak double definition – passes / succeeds in a study by the minimal acceptability).
Happy enough to get it finished eventually, but feel that I GOT BY rather than ‘passed with honours’ !