Roman poet loses his third watch (5)
I believe the answer is:
vigil
'watch' is the definition.
(to keep vigil is to keep watch)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for vigil that I've seen before include "Period of 9" , "Watchful night" , "Through the night, one kept" , "observance" , "Night service; watch" .)