Quantity of beer from vat a doctor knocked over (7)
I believe the answer is:
tankard
'quantity of beer' is the definition.
'vat a doctor knocked over' is the wordplay.
'vat' becomes 'tank' (I've seen this before).
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'knocked over' shows that the letters should be reversed in order.
'dr' in reverse letter order is 'rd'.
'tank'+'a'+'rd'='TANKARD'
'from' is the link.
(Other definitions for tankard that I've seen before include "Drinking vessel with a handle" , "Drank at (anag)" , "Beer-glass" , "Tall beer mug" , "Beer-mug" .)