Conclude hydrocarbon ultimately needs oxygen for conflagration (7)
I believe the answer is:
inferno
'conflagration' is the definition.
(synonyms)
'conclude hydrocarbon ultimately needs oxygen' is the wordplay.
'conclude' becomes 'infer' (inferring is a kind of concluding).
'ultimately' suggests the final letters.
'needs' says to put letters next to each other.
'oxygen' becomes 'o' (atomic symbol for oxygen).
The final letter of 'hydrocarbon' is 'n'.
'infer'+'n'+'o'='INFERNO'
'for' acts as a link.
(Other definitions for inferno that I've seen before include "(Fiery) Hell" , "Terrible fire" , "blazer?" , "Huge conflagration" , "Big fire" .)