Plant liable possibly to retain oxygen (7)
I believe the answer is:
lobelia
'plant' is the definition.
(lobelia is a kind of plant)
'liable possibly to retain oxygen' is the wordplay.
'possibly' indicates an anagram.
'to retain' means one lot of letters goes inside another.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'liable' is an anagram of 'lbelia'.
'lbelia' going around 'o' is 'LOBELIA'.
(Other definitions for lobelia that I've seen before include "Common garden flower" , "I lob ale (anag)" , "Bloomer" , "Boil ale to get the flower" , "Popular bedding plant" .)