Upset Liberal in love (3)
I believe the answer is:
nil
'love' is the definition.
(I've seen this before)
'upset liberal in' is the wordplay.
'upset' says the letters should be written backwards.
'liberal' becomes 'l' (abbreviation).
'l'+'in'='lin'
'lin' written backwards gives 'NIL'.
(Other definitions for nil that I've seen before include "Latin zero" , "Nothing - in Latin" , "Love" , "Nothing, naught" , "Non-existent" .)