Key spies blend in somehow, casing area (13)
I believe the answer is:
indispensable
'key' is the definition.
(thesaurus)
'spies blend in somehow casing area' is the wordplay.
'somehow' indicates anagramming the letters.
'casing' is an insertion indicator (I've seen this in other clues).
'area' becomes 'a' (maths abbreviation).
'spies'+'blend'+'in'='spiesblendin'
'spiesblendin' with letters rearranged gives 'indispensble'.
'indispensble' going around 'a' is 'INDISPENSABLE'.
(Other definitions for indispensable that I've seen before include "Essential" , "Absolutely necessary" , "Vitally necessary" .)