Worker has it, he is on island opposite (10)
I believe the answer is:
antithesis
'opposite' is the definition.
(I've seen this before)
'worker has it he is on island' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'has' means one lot of letters go next to another.
'he is' becomes 'hes'.
'on' is a charade indicator (letters next to each other).
'island' becomes 'is'.
'ant'+'it'+'hes'+'is'='ANTITHESIS'
(Other definitions for antithesis that I've seen before include "Contrast between two things" , "Direct opposite" , "Opposition" , "Opposite; contrasting idea" , "opposing view" .)