Oxygen absorbed by beer plant (4)
I believe the answer is:
aloe
'plant' is the definition.
(aloe is a kind of plant)
'oxygen absorbed by beer' is the wordplay.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'absorbed by' means one lot of letters goes inside another.
'beer' becomes 'ale' (I've seen this before**).
'o' inserted within 'ale' is 'ALOE'.
(Other definitions for aloe that I've seen before include "Fleshy-leaved plant" , "Succulent plant (4) 5532" , "Plant source of purgative drug" , "O, ale with bitter plant" , "Succulent plant (may be bitter!)" .)