Couples pop by tax department in USA (5)
I believe the answer is:
pairs
'couples' is the definition.
('pair' can be a synonym of 'couple')
'pop by tax department in usa' is the wordplay.
'pop' becomes 'pa' (both can mean father).
'by' says to put letters next to each other.
'tax department in usa' becomes 'IRS' (abbreviation for Internal Revenue Service).
'pa'+'irs'='PAIRS'
(Other definitions for pairs that I've seen before include "Couples" , "golfing matches" , "Duos" .)