Spoil one purchasing can for cocktail (7)
I believe the answer is:
martini
'cocktail' is the definition.
(martini is a kind of cocktail)
'spoil one purchasing can' is the wordplay.
'spoil' becomes 'mar' (synonyms).
'one' becomes 'i' (Roman numeral).
'purchasing' is an insertion indicator (some letters take possession of others).
'can' becomes 'tin' (synonyms).
'mar'+'i'='mari'
'mari' enclosing 'tin' is 'MARTINI'.
'for' acts as a link.
(Other definitions for martini that I've seen before include "alcoholic drink" , "Alternative name for Simone Memmi, pre-Renaissance Italian painter" , "obtained from a bottle" , "rifle" , "Cocktail of gin and dry vermouth" .)