It's crazy wearing hair that's artificial (3-4)
I believe the answer is:
man-made
'artificial' is the definition.
(thesaurus)
'it's crazy wearing hair' is the wordplay.
'it's crazy' becomes 'mad' (synonyms).
'wearing' is an insertion indicator.
'hair' becomes 'mane' (mane is a kind of hair).
'mad' going within 'mane' is 'MAN-MADE'.
'that's' acts as a link.
(Other definitions for man-made that I've seen before include "Caused by humans" , "Constructed by humans, not by nature" , "Artificial, synthetic" .)