Set off with fellow amphibians (4)
I believe the answer is:
efts
'amphibians' is the definition.
(eft is a kind of amphibian)
'set off with fellow' is the wordplay.
'off with' is an anagram indicator.
'fellow' becomes 'F' (abbreviation - of a society etc.).
'set' mixed up with 'f' makes 'EFTS'.
(Other definitions for efts that I've seen before include "Amphibians" , "Kind of newts" , "Baby newts" , "Old lizards" .)