ACROSS
6. <substantial> HEARTY = TY after HEAR
8. <make use of> EMPLOY = EM PLOY
9. *£*NICOLA*£* = NONCANONICAL <outside the official version of events>
10. SIGNALER* = SLANGIER <with more jargon>
12. <singer> ALTO = ALT + _O
13. <fool> B_ ZOO*
15. (CHEAP YET)* = EYEPATCH <accessory worn on the face>
17. <admit error> EAT HUMBLE PIE = EA THUMB L_ (E PI E)
19. <bare minimum> FEWEST = FE + WEST
20. <this makes CUBED MELON> ROT TEN = ROTTEN <mouldy?>

DOWN
1. ←LOOTER← = RETOOL <to get new equipment>
2. <religious song> HYMN = "HIM"
3. <nanny> WET-NURSE = (SURE WENT)*
4. <one investigating> SPECIAL AGENT = (EATING-PLACES)*
5. <miscellaneous collection> JOB LOT = (JOB, LOT)
7. (..e)RE CON NO IT RES(p..) = RECONNOITRES <does research ahead of time>
11. (..n)INE QUIT Y(o..) = INEQUITY <unfair treatment>
14. _OXE_ + YES = OXEYES <flowers>
16. <herbs> CHIVES = C_ HIVES
18. <report> BARK = <part of log>