Fellow staggers round firm engaged in making formal wear (5,5)

'formal wear' is the definition.

'fellow staggers round firm engaged in' is the wordplay.
'fellow' becomes 'f' (abbreviation - of a society etc.).
'staggers' becomes 'rocks' (both can mean to shock greatly).
'round' is an insertion indicator.
'firm' becomes 'co' (a firm is a company).
'engaged in' becomes 'at' (I've seen this in other clues**).
'co'+'at'='coat'
'rocks' placed around 'coat' is 'rockcoats'.
'f'+'rockcoats'='FROCK-COATS'

'making' is the link.