Dress up and talk big (4)
I believe the answer is:
brag
'talk big' is the definition.
(I've seen this in another clue)
'dress up' is the wordplay.
'dress' becomes 'garb' (synonyms).
'up' is a reversal indicator (in down clue: letters go upwards).
'garb' backwards is 'BRAG'.
'and' is the link.
(Other definitions for brag that I've seen before include "Show off, crow" , "Talk boastfully" , "boast; form of poker" , "Idle talk" , "'Show off, boast (4)'" .)