Hour with Oscar in local, hammered: the cause? (7)
I believe the answer is:
alcohol
'cause?' is the definition.
(I know that alcohol is or was a cause)
'hour with oscar in local hammered' is the wordplay.
'hour' becomes 'h' (abbreviation).
'with' means one lot of letters go next to another.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' indicates putting letters inside.
'hammered' indicates an anagram (slang term for drunk).
'h'+'o'='ho'
'local' is an anagram of 'alcol'.
'ho' going within 'alcol' is 'ALCOHOL'.
'the' is the link.
(Other definitions for alcohol that I've seen before include "Intoxicating liquid" , "Drug" , "The hard stuff" , "Source of intoxication" , "solvent" .)