The difference making one hiss at one’s former partner? (3)
I believe the answer is:
sex
'the difference making one hiss' is the definition.
I can't tell whether this defines the answer.
'one's former partner?' is the wordplay.
'one's' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'former partner?' becomes 'ex'.
's'+'ex'='SEX'
'at' acts as a link.
Can you help me to learn more?
(Other definitions for sex that I've seen before include "Hanky-panky" , "Gender" , "This symbol is epitome of glamour" , "VI (Lat.); m. or f." , "Expert in the treatment of coital problems" .)