Head of costumes fitting first class number for lead (7)
I believe the answer is:
captain
'lead' is the definition.
(I know that captain is a more specific form of lead)
'head of costumes fitting first class number' is the wordplay.
'head of' says to take the initial letters.
'fitting' becomes 'apt' (both mean appropriate).
'first class' becomes 'ai' (A1 - top quality).
'number' becomes 'n' (n can represent any number).
The first letter of 'costumes' is 'c'.
'c'+'apt'+'ai'+'n'='CAPTAIN'
'for' is the link.
(Other definitions for captain that I've seen before include "American police officer" , "US policeman" , "Ship's commander" , "Is Pat Cain in charge of the ship?" , "eg Hornblower" .)