Amphibian takes oxygen in small amount (4)
I believe the answer is:
toad
'amphibian' is the definition.
(I know that toad is a type 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' ('a tad' is a small amount).
'o' placed inside 'tad' is 'TOAD'.
'takes' acts as a link.
(Other definitions for toad that I've seen before include "Dangerous driver" , "Wind in Willows hall-owner" , "Warty creature" , "Ancestral hall on River Bank" , "Froglike animal or repulsive person" .)