Feels unwell (4)
I believe the answer is:
ails
'feels unwell' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ails that I've seen before include "Is sick" , "Is unwell - sail away" , "Is ill or uneasy - sail away" , "Troubles; afflicts" , "languishes" .)