Expedition heading for Sumatra, a distant island (6)
I believe the answer is:
safari
'expedition' is the definition.
(safari is a kind of expedition)
'heading for sumatra a distant island' is the wordplay.
'heading for' indicates taking the first letters.
'a distant' becomes 'afar' (I've seen this before).
'island' becomes 'i'.
The first letter of 'sumatra' is 's'.
's'+'afar'+'i'='SAFARI'
(Other definitions for safari that I've seen before include "Hunting trip" , "Hunting expedition (in Africa)" , "Overland journey to look for wild animals" , "African hunting expedition" , "Animal-viewing trip" .)