Creep into cocktail party (9)
I believe the answer is:
reception
'party' is the definition.
(reception is a kind of party)
'creep into cocktail' is the wordplay.
'cocktail' is an anagram indicator.
'creep'+'into'='creepinto'
'creepinto' with letters rearranged gives 'RECEPTION'.
(Other definitions for reception that I've seen before include "A social event" , "First class" , "at home" , "drinks party" , "Entrance for office" .)