Defender, about 50, seen before match in formal evening dress (5,3)
I believe the answer is:
black tie
'evening dress' is the definition.
(black tie is a kind of evening dress)
'defender about 50 seen before match' is the wordplay.
'defender' becomes 'back' (left-backs and right-backs are football defenders).
'about' is an insertion indicator.
'50' becomes 'l' (Roman numeral).
'seen before' means one lot of letters go next to another.
'match' becomes 'tie' (tie can mean a sporting fixture).
'back' going around 'l' is 'black'.
'black'+'tie'='BLACK-TIE'
'in formal' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for black tie that I've seen before include "It's worn with a dinner jacket" , "Formal evening dress for men" , "expensive party-going habit" , "Formal (neck)wear" , "Formal evening wear for men" .)