Amusing nobleman backed left (5)
I believe the answer is:
droll
'amusing' is the definition.
(I've seen this before)
'nobleman backed left' is the wordplay.
'nobleman' becomes 'lord' (synonyms).
'backed' says the letters should be written backwards.
'left' becomes 'L' (common abbreviation).
'lord' backwards is 'drol'.
'drol'+'l'='DROLL'
(Other definitions for droll that I've seen before include "Quaintly amusing" , "Drily amusing" , "Amusing in offbeat way" , "amusing, in a strange way" , "Humorous or whimsical" .)