Formal wear? It may be your call (5)
I believe the answer is:
tails
I believe this is a double definition.
'formal wear?' is the first definition.
(as in tail-suits)
'it may be your call' is the second definition.
(as in calling a coin toss heads or tails)
(Other definitions for tails that I've seen before include "Full evening dress for men" , "Follows closely" , "Coin-tossing call" , "A possible fall of the pound" , "as opposed to heads" .)