Mum’s in shed, completely drunk (7)
I believe the answer is:
smashed
'completely drunk' is the definition.
'smashed' can be an answer for 'drunk' (I've seen this in another clue). I am not certain of the 'completely' bit.
'mum's in shed' is the wordplay.
'mum's' becomes 'ma's' (both can mean mother).
'in' is an insertion indicator.
'mas' inserted inside 'shed' is 'SMASHED'.
Can you help me to learn more?
(Other definitions for smashed that I've seen before include "Drunk (as a broken bottle?)" , "Affected by drug" , "Vandalised" , "bust?" , "Broken in pieces" .)