Whimsical Liberal peer sent up (5)
I believe the answer is:
droll
'whimsical' is the definition.
(I've seen this before)
'liberal peer sent up' is the wordplay.
'liberal' becomes 'l' (abbreviation).
'peer' becomes 'lord' (lord is a type of peer).
'sent up' says the letters should be written in reverse.
'l'+'lord'='llord'
'llord' backwards is 'DROLL'.
(Other definitions for droll that I've seen before include "Funny" , "Amusing - jester" , "Dry" , "Clownish" , "Humorous or whimsical" .)