Since the luau's been arranged, like always (2,5)
I believe the answer is:
as usual
'like always' is the definition.
(I've seen this in another clue)
'since the luau's been arranged' is the wordplay.
'since' becomes 'as' (both can mean 'because').
'been arranged' is an anagram indicator.
'luaus' is an anagram of 'usual'.
'as'+'usual'='AS USUAL'
(Other definitions for as usual that I've seen before include "Predictably" , "In line with practice" , "'In the normal way (2,5)'" , "Customarily" , "In the normal way, habitually" .)