Key sale set in ground (9)
I believe the answer is:
essential
'key' is the definition.
(thesaurus)
'sale set in ground' is the wordplay.
'ground' is an anagram indicator (grind the letters ).
'sale'+'set'+'in'='salesetin'
'salesetin' is an anagram of 'ESSENTIAL'.
(Other definitions for essential that I've seen before include "something necessary" , "absolutely neccessary?" , "Basic or necessary" , "must-have item" , "organic" .)