Misplace medium Hebridean island (6)
I believe the answer is:
Here is my best explanation:
'misplace' is the definition.
('mislay' can be a synonym of 'misplace')
'medium hebridean island' is the subsidiary indication.
'medium' becomes 'm' (as in UK clothing sizes).
'hebridean island' becomes 'islay' (I've seen this before).
(Other definitions for mislay that I've seen before include "Lose temporarily", "Put in wrong place, lose temporarily", "Lose track of", "Put something aside and forget where", "Be unable to find", "Lose (an item)", "Set in wrong location", "Lose - put in wrong place", "Put (something) where it can't be found" and "Lose (something)".)