Establish insomnia (drowsiness has returned) (6)
I believe the answer is:
ordain
'establish' is the definition.
(ordaining is a kind of establishing)
'insomnia drowsiness has returned' is the wordplay.
'has' indicates a hidden word.
'returned' says the letters should be written backwards.
'niadro' can be found hidden inside 'insomnia drowsiness'.
'niadro' in reverse letter order is 'ORDAIN'.
(Other definitions for ordain that I've seen before include "Lay down (law)" , "Decree or appoint to priesthood" , "Either prescribe" , "Decree authoritatively" , "Invest with ministerial authority" .)