The measure of a pious testament (4)
I believe the answer is:
pint
'the measure' is the definition.
(pint is a kind of measure)
'a pious testament' is the wordplay.
'a pious' becomes 'pi'.
'testament' becomes 'nt'.
'pi'+'nt'='PINT'
'of' acts as a link.
(Other definitions for pint that I've seen before include "Standard measure of Guinness" , "Liquid meassure - four gills" , "Unit of beer" , "small amount" , "Liquid measure" .)