Lease isn't changed producing key (9)
I believe the answer is:
essential
'key' is the definition.
(I've seen this before)
'lease isn't changed' is the wordplay.
'changed' indicates an anagram.
'lease'+'isnt'='leaseisnt'
'leaseisnt' anagrammed gives 'ESSENTIAL'.
'producing' acts as a link.
(Other definitions for essential that I've seen before include "Key" , "organic" , "as oil might be" , "So is certain oil-production" , "Type of oil" .)