Big fire I observed primarily consuming northern plant (7)
I believe the answer is:
inferno
'big fire' is the definition.
(I've seen this before)
'i observed primarily consuming northern plant' is the wordplay.
'primarily' suggests taking the first letters.
'consuming' indicates putting letters inside.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'plant' becomes 'fern' (fern is a kind of plant**).
The initial letter of 'observed' is 'o'.
'i'+'o'='io'
'n'+'fern'='nfern'
'io' going around 'nfern' is 'INFERNO'.
(Other definitions for inferno that I've seen before include "Huge conflagration" , "Furious" , "Great fire" , "Big fire" , "Hell - conflagration" .)