United without captain in days gone by (4)
I believe the answer is:
once
'in days gone by' is the definition.
(I've seen this in another clue)
'united without captain' is the wordplay.
'united' becomes 'one' (as in a phrase like 'we are one').
'without' is an insertion indicator ('without' can be similar in meaning to 'outside').
'captain' becomes 'c' (this could be a standard abbreviation of which I'm unaware).
'one' enclosing 'c' is 'ONCE'.
Can you help me to learn more?
(Other definitions for once that I've seen before include "Long ago" , "... upon a time for the storyteller" , "yesterday" , "Singly" , "in days of yore?" .)