Thump boyfriend admitting condition (4)
I believe the answer is:
biff
'thump' is the definition.
(thesaurus)
'boyfriend admitting condition' is the wordplay.
'boyfriend' becomes 'bf' (common abbreviation for boyfriend).
'admitting' means one lot of letters goes inside another (admit can mean to welcome in or accept).
'condition' becomes 'if' (as in 'a big if').
'bf' placed around 'if' is 'BIFF'.
(Other definitions for biff that I've seen before include "strike" , "Punch" , "Hard blow with fist" .)