Amphibian taking oxygen in small amount (4)
I believe the answer is:
toad
'amphibian' is the definition.
(toad is a kind of amphibian)
'oxygen in small amount' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'in' is an insertion indicator.
'small amount' becomes 'tad' (I've seen this before).
'o' put within 'tad' is 'TOAD'.
'taking' acts as a link.
(Other definitions for toad that I've seen before include "Hateful person; animal" , "Wind in Willows hall-owner" , "driver of reckless repute" , "Small amphibian" , "species of amphibian" .)