One adopted by average couple (4)
I believe the answer is:
pair
'couple' is the definition.
(I know that couple can be written as pair)
'one adopted by average' is the wordplay.
'one' becomes 'i' (Roman numeral).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'average' becomes 'par' (average score in golf).
'i' going inside 'par' is 'PAIR'.
(Other definitions for pair that I've seen before include "Cereal used as food" , "Two of a kind, as in cards" , "flight of stairs" , "players of duet?" , "E.g. two aces; mate" .)