Amphibian of hefty interior (3)
I believe the answer is:
eft
'amphibian' is the definition.
(I know that eft is a type of newt)
'of hefty interior' is the wordplay.
'of' says the answer is hidden in the clue.
'EFT' can be found hidden inside 'hefty interior'.
(Other definitions for eft that I've seen before include "Slimy creature" , "Pond creature" , "Young newt" , "Small amphibian" , "Swimmer with feeble legs" .)