Instruct nurses to cover daytime (9)
I believe the answer is:
enlighten
'instruct' is the definition.
(enlightening is a kind of instructing)
'nurses to cover daytime' is the wordplay.
I cannot really see how this works, but
'daytime' could be 'light' (as opposed to night-time, the dark) and 'light' is located in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for enlighten that I've seen before include "Brief" , "Apprise" , "Impart knowledge to" , "Explain" , "Free from ignorance" .)