Declare it's proper to drink cola, shaken or stirred (8)
I believe the answer is:
proclaim
'declare' is the definition.
(proclaiming is a kind of declaring)
'proper to drink cola shaken or stirred' is the wordplay.
'proper' becomes 'prim' (similar in meaning).
'to drink' means one lot of letters goes inside another.
'shaken or stirred' indicates anagramming the letters.
'cola' with letters rearranged gives 'ocla'.
'prim' placed around 'ocla' is 'PROCLAIM'.
'it's' is the link.
(Other definitions for proclaim that I've seen before include "Speak out" , "Herald" , "Announce officially or declare publicly" , "advertise" , "Declare formally" .)