Anything northern rejected for couple (3)
I believe the answer is:
two
'couple' is the definition.
(thesaurus)
'anything northern rejected' is the wordplay.
'anything northern' becomes 'owt' (northern dialect term for 'anything').
'rejected' shows that the letters should be reversed in order.
'owt' backwards is 'TWO'.
'for' acts as a link.
(Other definitions for two that I've seen before include "'It takes ? to tango'" , "Number in company, some say" , "Pair, deuce" , "Even number" , "Prime number" .)