Lease isn't negotiated for key (9)
I believe the answer is:
essential
'for key' is the definition.
(I've seen this in another clue)
'lease isn't negotiated' is the wordplay.
'negotiated' is an anagram indicator.
'lease'+'isnt'='leaseisnt'
'leaseisnt' is an anagram of 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "Key" , "Type of oil" , "'Absolutely necessary, required (9)'" , "organic" , "So is certain oil-production" .)