Formal evening dress for men (5,3)
I believe the answer is:
black tie
'formal evening dress for men' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for black tie that I've seen before include "Formal evening wear for men" , "expensive party-going habit" , "Formal (neck)wear" , "It's worn with a dinner jacket" , "Formal attire" .)