Look sulky when up to no good? (4)
I believe the answer is:
pout
'look sulky' is the definition.
(I've seen this before)
'up to no good?' is the wordplay.
'no good?' is an anagram indicator (I've seen 'no good' mean this (no good can mean faulty or defective)).
'up'+'to'='upto'
'upto' with letters rearranged gives 'POUT'.
'when' is the link.
(Other definitions for pout that I've seen before include "prepare for selfie?" , "Look sulky" , "Push lips forward in sulky expression" , "Sulky (or sexy?) expression" , "Push out the lips (alluringly?)" .)