Amphibian remaining headless (3)
I believe the answer is:
eft
'amphibian' is the definition.
(eft is a kind of amphibian)
'remaining headless' is the wordplay.
'remaining' becomes 'left' ('left' can be a synonym of 'remaining').
'headless' means to remove the first letter.
'left' with its initial letter removed is 'EFT'.
(Other definitions for eft that I've seen before include "Swimmer with feeble legs" , "Pond creature" , "Slimy creature" , "Newt (archaic)" , "Little creature" .)