Alcohol diligently on vacation put on cereal (6)
I believe the answer is:
brandy
'alcohol' is the definition.
(brandy is a kind of alcohol)
'diligently on vacation put on cereal' is the wordplay.
'on vacation' suggests removing the centre.
'put on' is a charade indicator (letters next to each other).
'cereal' becomes 'bran' (type of cereal).
'diligently' with its middle taken out is 'dy'.
'dy' after 'bran' is 'BRANDY'.
(Other definitions for brandy that I've seen before include "Samuel Johnson's drink fit for heroes" , "Strong alcoholic spirit" , "Alcoholic drink" , "Cognac, perhaps" , "Strong liquor" .)