Dress up showing swagger (4)
I believe the answer is:
brag
'swagger' is the definition.
(thesaurus)
'dress up' is the wordplay.
'dress' becomes 'garb' ('garb' can be a synonym of 'dress').
'up' says the letters should be written backwards (in down clue: letters go upwards).
'garb' backwards is 'BRAG'.
'showing' is the link.
(Other definitions for brag that I've seen before include "game of bluff" , "Boast, exaggerate" , "Card game, like poker" , "Boast ? card-game" , "Boast about garb" .)