Grass covered by mould? (8)
I believe the answer is:
informer
'grass' is the definition.
(grass can mean a police informant)
'covered by mould?' is the wordplay.
'covered' becomes 'in' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'mould?' becomes 'former' (a mould shapes or forms something).
'in'+'former'='INFORMER'
(Other definitions for informer that I've seen before include "Whistle-blower" , "Grass, nark, canary" , "(Maybe super)grass" , "A grass, one who tells all to the police" , "who may betray someone?" .)