Dull not having alcohol over bit of cereal (6)
I believe the answer is:
dreary
'dull' is the definition.
(similar in meaning)
'not having alcohol over bit of cereal' is the wordplay.
'not having alcohol' becomes 'dry' (dry can mean abstaining from alcohol).
'over' means one lot of letters goes inside another.
'bit of cereal' becomes 'ear' (as in an ear of wheat or another cereal grain).
'dry' enclosing 'ear' is 'DREARY'.
(Other definitions for dreary that I've seen before include "Dark" , "Dismal, funereal" , "Dull and depressing" , "Drab and depressing" , "Depressingly dull" .)