Party King eyed regularly with drink (5-2)
I believe the answer is:
knees-up
'party' is the definition.
(I have seen 'Lively party' mean 'knees up' so perhaps 'party' could also mean 'knees up')
'king eyed regularly with drink' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'with' is a charade indicator (letters next to each other).
'drink' becomes 'sup' (to sup is to drink).
'king'+'eyed'='kingeyed'
The alternating letters of 'kingeyed' are 'knee'.
'knee'+'sup'='KNEES-UP'
(Other definitions for knees-up that I've seen before include "Dance with Mother Brown" , "Riotous dance - party" , "Informal party with dancing" , "Lively informal party" , "Informal shindig (with Mother Brown?)" .)