Spirit: type unknown? (6)
I believe the answer is:
brandy
'spirit' is the definition.
(alcoholic spirit)
'type unknown?' is the wordplay.
'type' becomes 'brand' (type of product).
'unknown?' becomes 'y' (algebra).
'brand'+'y'='BRANDY'
(Other definitions for brandy that I've seen before include "Of a legal entity acting as an individual" , "Grape wine spirit" , "Strong drink distilled from wine" , "Distilled alcohol" , "Samuel Johnson's drink fit for heroes" .)