Nobleman upset Liberal? That's funny (5)
I believe the answer is:
droll
'funny' is the definition.
(thesaurus)
'nobleman upset liberal?' is the wordplay.
'nobleman' becomes 'lord' ('lord' can be a synonym of 'nobleman').
'upset' says the letters should be written backwards.
'liberal?' becomes 'l' (abbreviation).
'lord' reversed gives 'drol'.
'drol'+'l'='DROLL'
'that's' is the link.
(Other definitions for droll that I've seen before include "Drily amusing" , "Dry" , "Amusing - jester" , "Quaintly amusing" , "Whimsically comical" .)