An amphibian went crazy (4)
I believe the answer is:
newt
'an amphibian' is the definition.
(newt is a kind of amphibian)
'went crazy' is the wordplay.
'crazy' is an anagram indicator.
'went' with letters rearranged gives 'NEWT'.
(Other definitions for newt that I've seen before include "Semiaquatic amphibian" , "Medical worker" , "animal" , "found in pond!" , "Small, lizard-like amphibian" .)