Whimsical, somewhat topsy-turvy belief (3)
I believe the answer is:
ism
'belief' is the definition.
(the -ism suffix in the names of belief systems)
'whimsical somewhat topsy-turvy' is the wordplay.
'somewhat' indicates a hidden word.
'topsy turvy' says the letters should be written backwards.
'msi' is hidden within 'whimsical'.
'msi' backwards is 'ISM'.
(Other definitions for ism that I've seen before include "Distinctive theory" , "Informally, any doctrine or practice" , "Stage performance of singing" , "Informally a doctrine, or suffix for one" , "Any system of beliefs" .)