Amphibian a fox swallows (4)
I believe the answer is:
toad
'amphibian' is the definition.
(toad is a kind of amphibian)
'a fox swallows' is the wordplay.
'fox' becomes 'tod' (historic term).
'swallows' means one lot of letters goes inside another.
'a' placed into 'tod' is 'TOAD'.
(Other definitions for toad that I've seen before include "Wind in Willows hall-owner" , "Small amphibian" , "Froglike animal or repulsive person" , "Hateful person; animal" , "Common amphibian" .)