Voluntary work involving keys will engross island (8)
I believe the answer is:
optional
'voluntary' is the definition.
(I've seen this before)
'work involving keys will engross island' is the wordplay.
I cannot quite understand how this works, but
'work' could be 'op' (common abbreviation for 'opus') and 'op' is located in the answer.
'island' could be 'iona' (Scottish island) and 'iona' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for optional that I've seen before include "Too plain (anag.)" , "Platoon I may leave to choice" , "Platoon I join as a matter of choice" , "Unforced" , "Voluntary - not required" .)