Lose many a Scottish island! (6)
I believe the answer is:
mislay
'lose' is the definition.
('mislay' can be a synonym of 'lose')
'many a scottish island' is the wordplay.
'many' becomes 'm'.
'a scottish island' becomes 'islay' (I've seen this before).
'm'+'islay'='MISLAY'
(Other definitions for mislay that I've seen before include "Put (something) where it can't be found" , "Set in wrong location" , "Lose (an item)" , "Lose (something)" , "Lose track of" .)