I knock back about a couple (4)
I believe the answer is:
pair
'a couple' is the definition.
('pair' can be a synonym of 'couple')
'i knock back about' is the wordplay.
'knock' becomes 'rap' ('rap' can be a synonym of 'knock'**).
'back' shows that the letters should be reversed in order.
'about' is an insertion indicator.
'rap' in reverse letter order is 'par'.
'i' put inside 'par' is 'PAIR'.
(Other definitions for pair that I've seen before include "Twosome" , "Put together - two of a kind" , "E.g. two aces; mate" , "Agreement to avoid division" , "flight of stairs" .)