Toast four on Arabic island in mock serenade (9)
I believe the answer is:
charivari
'mock serenade' is the definition.
(I've seen this before)
'toast four on arabic island' is the wordplay.
'toast' becomes 'char' (I've seen this in another clue).
'four' becomes 'IV' (Roman numerals).
'on' says to put letters next to each other.
'arabic' becomes 'Ar.' (abbreviation in Chambers).
'island' becomes 'i' (geographical abbreviation).
'char'+'iv'+'ar'+'i'='CHARIVARI'
'in' acts as a link.
(Other definitions for charivari that I've seen before include "not a musical sound" , "Hubbub" , "Confused noise" , "Noisy welcome" , "uproar" .)