Mother's in Slough getting drunk (7)
I believe the answer is:
smashed
'drunk' is the definition.
(thesaurus)
'mother's in slough' is the wordplay.
'mother's' becomes 'mas' (ma is a kind of mother).
'in' indicates putting letters inside.
'slough' becomes 'shed' (both can mean to remove or lose).
'mas' placed into 'shed' is 'SMASHED'.
'getting' acts as a link.
(Other definitions for smashed that I've seen before include "Broke; drunk (colloq.)" , "Vandalised" , "bust?" , "Affected by drug" , "Broken in pieces" .)