Fat Inca turned into extremist (7)
I believe the answer is:
fanatic
'extremist' is the definition.
(I've seen this before)
'fat inca turned' is the wordplay.
'turned' is an anagram indicator.
'fat'+'inca'='fatinca'
'fatinca' with letters rearranged gives 'FANATIC'.
'into' is the link.
(Other definitions for fanatic that I've seen before include "enthusiastic" , "Freak" , "Person filled with excessive zeal for cause" , "Bigot - enthusiast" , "Over-enthusiastic person" .)