Amusing nobleman backed Liberal (5)
I believe the answer is:
droll
'amusing' is the definition.
(I've seen this before)
'nobleman backed liberal' is the wordplay.
'nobleman' becomes 'lord' (synonyms).
'backed' is a reversal indicator.
'liberal' becomes 'l' (abbreviation).
'lord' in reverse letter order is 'drol'.
'drol'+'l'='DROLL'
(Other definitions for droll that I've seen before include "Humorous or whimsical" , "Oddly amusing or comical" , "Comic" , "Whimsically comical" , "Drily witty" .)