Setter adopted by average couple (4)
I believe the answer is:
pair
'couple' is the definition.
(synonyms)
'setter adopted by average' is the wordplay.
'setter' becomes 'I' (the person who set this crossword).
'adopted by' indicates putting letters inside (I've seen this in other clues).
'average' becomes 'par' (average score in golf).
'i' put into 'par' is 'PAIR'.
(Other definitions for pair that I've seen before include "item" , "Twosome" , "Duo" , "how braces come" , "Couple - wed" .)