Equal with one couple (4)
I believe the answer is:
'couple' is the definition.
(I know that couple can be written as pair)
'equal with one' is the wordplay.
'equal' becomes 'par'.
'with' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'par' placed around 'i' is 'PAIR'.
(Other definitions for pair that I've seen before include "Two of a similar type" , "flight of stairs" , "E.g. two aces; mate" , "Cereal used as food" , "Put together - two of a kind" .)