Amphibian needs 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' means one lot of letters goes inside another.
'small amount' becomes 'tad' (I've seen this before).
'o' going within 'tad' is 'TOAD'.
'needs' acts as a link.
(Other definitions for toad that I've seen before include "Warty creature" , "not found in Ireland" , "driver of reckless repute" , "species of amphibian" , "Ugly creature, jewel in head (AYLI)" .)