Dress crow up (4)
I believe the answer is:
garb
'dress' is the definition.
(I know that dress can be written as garb)
'crow up' is the wordplay.
'crow' becomes 'brag' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'brag' reversed gives 'GARB'.
(Other definitions for garb that I've seen before include "Clothe" , "costume" , "Clothes to brag about" , "something to wear" , "Style of clothing" .)