Deduce friend not quite crazy (5)
I believe the answer is:
infer
'deduce' is the definition.
(I know that deduce can be written as infer)
'friend not quite crazy' is the wordplay.
'not quite' means to remove the last letter (most but not all of the word).
'crazy' is an anagram indicator.
'friend' with its final letter taken off is 'frien'.
'frien' with letters rearranged gives 'INFER'.
(Other definitions for infer that I've seen before include "Take finer meaning from" , "Work out" , "Derive by reasoning" , "Insinuate" , "Extrapolate, conclude" .)