Fifteensquared

I seemed to solve this one in fits and starts; with groups of answers coming in short order, but punctuated by several lulls of a minute or two.  A smattering of nifty &lit type clues, with 13D almost being worth the entrance fee on its own.

Across
1 DISCREDIT – disc+r[un]+edit.
6 ALLOW – callow -c.
9 SPASM – s in spam. I echo the sentiments of the surface reading.
10 SEMBLANCE – se[e}+MB+lance.
11 ORDER PAPER
12 THUS -Ref Thus Spake Zarathustra (Also Sprach Z....) by Strauss. Aka that classical theme tune with the kettledrums in 2001 - A Space Odyssey.
14 EMIRATE - emigrate - g.
15 TORPEDO - (depositor)* -si.  "Without backing is" being an enterprisingly precise subtraction indicator.
17 OVERLAY - a in overly.
19 PLACATE - CA (chartered acc't) in plate.
20 GIST - g+1st.  The use of "first" to denote -ist has never really tickled my fancy.
22 PROMISSORY - pro+miss+[T]ory.
25 TOLERANCE – O+le in trance.
26 IRATE – pirate (“Silver, perhaps”) – p.   I particularly enjoyed “defaced” as the decapitation indicator.
27 DIARY – I,r in D+ay &lit. As a rule, I’m on the fence as to whether container-with-separate-contents clue should necessarily indicate the segregation. However, in this case  a strict interpretation of the wordplay could form an actual word (i.e. dairy).
28 STICKY END – (sent dicky)*. An innuendophile’s favourite “disaster”.

Down
1 DISCO – Is+C in do &lit.
2 STANDPIPE – (Pepsi and T)*.
3 REMARKABLY – mark+a b in rely.
4  DISEASE  – displease – pl[ace].
5 TEMPEST – met(<=) + pest.
6 ABLE – table -t.
7 LUNCH – launch -a.
8 WHETSTONE – (when set to)* &lit.
13 BREADSTICK – b+read+stick &lit. Magnificio!
14 ELONGATED – lon[e]+gate in Ed.
16 ELABORATE – labor in E+ate.
18 Y-FRONTS – Fr in (stony)*. “Stony broke” is a simple but amusing opportunity well-taken.
19 POMPEII – m(ass) in pope +II
21 SALSA – SA (sex appeal= “It”, again! Enough already!) + s in LA (Latin American, although that’s not an abbreviation I’ve ever encountered in day-to-day use).
23 YIELD – y for f in field.
24 FRAY – a in fry.