Expedition heading for Surtsey, a distant island (6)
I believe the answer is:
safari
'expedition' is the definition.
(I know that safari is a type of expedition)
'heading for surtsey 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' (geographical abbreviation).
The initial letter of 'surtsey' is 's'.
's'+'afar'+'i'='SAFARI'
(Other definitions for safari that I've seen before include "Big game expedition" , "[PARK] trip" , "Animal expedition" , "Hunting trip" , "Animal-viewing trip" .)