Clubs held by United — but not now? (4)
I believe the answer is:
once
'not now?' is the definition.
(at one time, but not any more)
'clubs held by united' is the wordplay.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'held by' means one lot of letters goes inside another.
'united' becomes 'one' (as in a phrase like 'we are one').
'c' put into 'one' is 'ONCE'.
'but' is the link.
(Other definitions for once that I've seen before include "In the old days" , "In former times" , "On a single (or past) occasion" , "... upon a time for the storyteller" , "Long ago" .)