Sticks up when smirking? (4)
I believe the answer is:
smug
'smirking?' is the definition.
(I've seen this before)
'sticks up' is the wordplay.
'sticks' becomes 'gums'.
'up' is a reversal indicator (in down clue: letters go upwards).
'gums' reversed gives 'SMUG'.
'when' acts as a link.
(Other definitions for smug that I've seen before include "Arrogantly self-satisfied" , "Self-satisfied, complacent" , "Irritatingly pleased with oneself" , "Showing excessive self-pride" , "Full of oneself" .)