Insect, I wager, is metamorphosing (6)
I believe the answer is:
earwig
'insect' is the definition.
(earwig is a kind of insect)
'i wager is metamorphosing' is the wordplay.
'is metamorphosing' indicates an anagram.
'i'+'wager'='iwager'
'iwager' is an anagram of 'EARWIG'.
(Other definitions for earwig that I've seen before include "A bug for Americans" , "Small insect with pincers at end of its body" , "Eavesdrop (informal)" , "little insect" , "Clawed insect" .)