Equal with one couple (4)
I believe the answer is:
pair
'couple' is the definition.
(synonyms)
'equal with one' is the wordplay.
'equal' becomes 'par'.
'with' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'par' enclosing '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" .)