Wicked rat swallowing potatoes? (8)
I believe the answer is:
smashing
'wicked' is the definition.
(both can informally mean excellent)
'rat swallowing potatoes?' is the wordplay.
'rat' becomes 'sing' (both can mean to inform on someone).
'swallowing' is an insertion indicator.
'potatoes?' becomes 'mash' (mashed potato).
'sing' enclosing 'mash' is 'SMASHING'.
(Other definitions for smashing that I've seen before include "2 [COOL] or 3 [DANDY] perhaps, possibly 20d [WICKED] or 22 [GEAR] !" , "Committing act of vandalism" , "Such destructive behaviour" , "Breaking violently into pieces" , "Producing tennis shot" .)