Clubs held by United in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(once upon a time)
'clubs held by united' is the wordplay.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'held by' is an insertion indicator.
'united' becomes 'one' (as in a phrase like 'we are one').
'c' going into 'one' is 'ONCE'.
(Other definitions for once that I've seen before include "One-time - one time" , "At some time past" , "In former times" , "In days gone by" , "in bygone era" .)