Dress up for game (4)
I believe the answer is:
brag
'for game' is the definition.
(I've seen this before)
'dress up' is the wordplay.
'dress' becomes 'garb' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'garb' in reverse letter order is 'BRAG'.
(Other definitions for brag that I've seen before include "Boast ? card-game" , "Boast, exaggerate" , "boast; form of poker" , "Shoot a line" , "Blow one's own trumpet" .)