This nutcase, I fancy, is passionate (12)
I believe the answer is:
enthusiastic
'passionate' is the definition.
('enthusiastic' is associated in meaning with 'passionate')
'this nutcase i fancy' is the wordplay.
'fancy' is an anagram indicator.
'this'+'nutcase'+'i'='thisnutcasei'
'thisnutcasei' anagrammed gives 'ENTHUSIASTIC'.
'is' acts as a link.
(Other definitions for enthusiastic that I've seen before include "Eager" , "Gung-ho" , "Showing keen interest" , "Ardent - thin suitcase (anag)" , "warm" .)