Certainly in drink -- so restrained (8)
I believe the answer is:
measured
'restrained' is the definition.
(I've seen this before)
'certainly in drink' is the wordplay.
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly').
'in' is an insertion indicator.
'drink' becomes 'mead' (mead is a kind of drink**).
'sure' going inside 'mead' is 'MEASURED'.
'so' is the link.
(Other definitions for measured that I've seen before include "Evaluated" , "Gauged - regulated - steady" , "Made sure (anag.)" , "Rhythmical, regular in movement" , "Sized; carefully chosen" .)