Say love stifles resistance for this reason (4)
I believe the answer is:
ergo
'for this reason' is the definition.
('ergo' is a Latin word meaning 'therefore')
'say love stifles resistance' is the wordplay.
'say' becomes 'eg' (both can mean 'for example').
'love' becomes 'o' (love means zero in tennis).
'stifles' indicates putting letters inside.
'resistance' becomes 'r' (electronics abbreviation).
'eg'+'o'='ego'
'ego' going around 'r' is 'ERGO'.
(Other definitions for ergo that I've seen before include "for this reason" , "Therefore (Latin)" , "'Cogito, ---- sum'" , "It follows" , "Therefore - what a goer" .)