A Martini stirred finally? No it's shaken as a rule (14)
I believe the answer is:
administration
'a rule' is the definition.
(thesaurus)
'a martini stirred finally? no it's shaken' is the wordplay.
'finally?' suggests the final letters (I've seen 'finally' mean this).
'shaken' indicates an anagram.
The last letter of 'stirred' is 'd'.
'martini'+'d'+'no'+'its'='martinidnoits'
'martinidnoits' anagrammed gives 'dministration'.
'a'+'dministration'='ADMINISTRATION'
'as' acts as a link.
(Other definitions for administration that I've seen before include "Running a business" , "government in power" .)