Guaranteed details, extra sparse (7)
I believe the answer is:
surplus
'sparse' is the definition.
Both the definition and answer are adjectives. Maybe you can see an association between them that I don't see?
'guaranteed details extra' is the wordplay.
'guaranteed details' becomes 'sur' (I am not sure about this - if you are sure you should believe this answer much more).
'extra' becomes 'plus' (plus can mean more than expected).
'sur'+'plus'='SURPLUS'
Can you help me to learn more?
(Other definitions for surplus that I've seen before include "Extra to requirements" , "superfluity" , "Excess, overstock" , "balance" , "Surfeit" .)