Reserve number one of eleven for clerk? (10)
I believe the answer is:
bookkeeper
'clerk?' is the definition.
(thesaurus)
'reserve number one of eleven' is the wordplay.
'reserve' becomes 'book' (booking is a kind of reserving).
'number one of eleven' becomes 'keeper' (football goal keepers normally wear the number 1).
'book'+'keeper'='BOOKKEEPER'
'for' is the link.
(Other definitions for bookkeeper that I've seen before include "Accounts official" , "Business occupation" , "who works to maintain balance" , "person responsible for financial oversight" , "accountant" .)