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' indicates an anagram.
'went' is an anagram of 'NEWT'.
(Other definitions for newt that I've seen before include "Semiaquatic amphibian" , "Medical worker" , "animal" , "found in pond!" , "Small, lizard-like amphibian" .)