Partner that is drunkard imbibing quietly (6)
I believe the answer is:
spouse
'partner' is the definition.
(I know that partner can be written as spouse)
'drunkard imbibing quietly' is the wordplay.
'drunkard' becomes 'souse' (souse can mean a drunken person).
'imbibing' means one lot of letters goes inside another.
'quietly' becomes 'p' (p for piano and quiet).
'souse' enclosing 'p' is 'SPOUSE'.
'that is' acts as a link.
(Other definitions for spouse that I've seen before include "Very hot and dry" , "old lady?" , "Possibly wife" , "Elizabethan dance" , "Other half" .)