One serving island in the sea, perhaps (6)
I believe the answer is:
waiter
'one serving' is the definition.
'island in the sea perhaps' is the wordplay.
'island' becomes 'i'.
'in the' means one lot of letters goes inside another.
'sea perhaps' becomes 'water' (sea is a kind of water).
'i' inserted into 'water' is 'WAITER'.
(Other definitions for waiter that I've seen before include "Restaurant worker" , "Steward" , "Restaurant employee" , "Herr Ober (Ger.)" , "Servant in bar" .)