Poet mislaying his third watch (5)
I believe the answer is:
vigil
'watch' is the definition.
(to keep vigil is to keep watch)
I cannot understand how the rest of the clue works.
(Other definitions for vigil that I've seen before include "Period of 9" , "Night-time watch" , "observance" , "Watchful night" , "Purposeful surveillance to guard or observe" .)