Small face is conceited (4)
I believe the answer is:
smug
'is conceited' is the definition.
(I've seen this in another clue)
'small face' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'face' becomes 'mug' (informal term for face).
's'+'mug'='SMUG'
(Other definitions for smug that I've seen before include "Full of oneself" , "Irritatingly pleased with oneself" , "Superior" , "goody-goody" , "Irritatingly self-satisfied" .)