Unit capturing cape in the past (4)
I believe the answer is:
once
'in the past' is the definition.
(once upon a time)
'unit capturing cape' is the wordplay.
'unit' becomes 'one' (synonyms).
'capturing' means one lot of letters goes inside another.
'cape' becomes 'c' (abbreviation used on maps).
'one' going around 'c' is 'ONCE'.
(Other definitions for once that I've seen before include "in days of yore?" , "in bygone days" , "Cone (anag.)" , "At some time past" , "In days past" .)