Certainly in drink but 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' (synonyms).
'in' indicates putting letters inside.
'drink' becomes 'mead' (mead is a kind of drink).
'sure' inserted inside 'mead' is 'MEASURED'.
'but' acts as a link.
(Other definitions for measured that I've seen before include "Sized; carefully chosen" , "Unhurried and with care and dignity" , "Calculated, quantified" , "Stately" , "Sober" .)