Vital spies blend in, adapting around America (13)
I believe the answer is:
indispensable
'vital' is the definition.
(similar in meaning)
'spies blend in adapting around america' is the wordplay.
'adapting' indicates anagramming the letters.
'around' indicates putting letters inside.
'america' becomes 'a' (common abbreviation - e.g. in organisation names).
'spies'+'blend'+'in'='spiesblendin'
'spiesblendin' anagrammed gives 'indispensble'.
'indispensble' going around 'a' is 'INDISPENSABLE'.
(Other definitions for indispensable that I've seen before include "Absolutely necessary" , "Essential" , "Vitally necessary" .)