Fail to find motorway covering Scottish island (6)
I believe the answer is:
Here is my best explanation:
'fail' is the definition.
Although both the answer and definition are verbs in their base form, I don't see how one could define the other.
'motorway covering scottish island' is the wordplay.
'motorway' becomes 'm'.
'covering' says to put letters next to each other (on top of in down clue).
'scottish island' becomes 'islay' (I've seen this before).
'to find' is the link.
(Other definitions for mislay that I've seen before include "Lose - put in wrong place", "Lose temporarily", "Put something aside and forget where", "Temporarily not find", "Put (something) where it can't be found", "Put in wrong place, lose temporarily", "Set in wrong location", "Lose (something)", "Lose (an item)" and "Lose track of".)