In plant, oxygen creates huge fire (7)
I believe the answer is:
inferno
'huge fire' is the definition.
(I've seen this before)
'in plant oxygen' is the wordplay.
'plant' becomes 'fern' (fern is a kind of plant).
'oxygen' becomes 'o' (atomic symbol for oxygen).
'in'+'fern'+'o'='INFERNO'
'creates' is the link.
(Other definitions for inferno that I've seen before include "Fierce fire" , "Huge conflagration" , "Large uncontrolled fire" , "Where Satan lives and sinners go" , "Hellish poem" .)