As right measure (5)
I believe the answer is:
quart
'measure' is the definition.
(quart is a kind of measure)
'as right' is the wordplay.
'as' becomes 'qua' (I've seen this before).
'right' becomes 'rt'.
'qua'+'rt'='QUART'
(Other definitions for quart that I've seen before include "A couple of pints" , "water may be measured in this" , "Liquid measure" , "The capacity" , "Two imperial pints" .)