Certain to get in alcoholic drink, carefully considered (8)
I believe the answer is:
measured
'carefully considered' is the definition.
(I've seen this before)
'certain to get in alcoholic drink' is the wordplay.
'certain' becomes 'sure' ('sure' can be a synonym of 'certain').
'to get in' indicates putting letters inside.
'alcoholic drink' becomes 'mead' (I've seen this before).
'sure' put into 'mead' is 'MEASURED'.
(Other definitions for measured that I've seen before include "Stately" , "Gauged - regulated - steady" , "Determine dimensions" , "Weighed" , "certain content of alcohol" .)