Formal attire and trendy related product (3,2)
I believe the answer is:
tie in
'related product' is the definition.
(I've seen this in another clue)
'formal attire and trendy' is the wordplay.
'formal attire' becomes 'tie'.
'and' is a charade indicator (letters next to each other).
'trendy' becomes 'in' ('in' can mean fashionable).
'tie'+'in'='TIE-IN'
(Other definitions for tie in that I've seen before include "Draw" , "Eg, film on book" , "Commercial connection" , "Correspondence" , "Something associated with film, say" .)