Is unwell (4)
I believe the answer is:
ails
'is unwell' is the definition.
('ail' can be a synonym of 'be unwell')
This is all the clue.
(Other definitions for ails that I've seen before include "Feels unwell" , "Troubles; afflicts" , "Is ill or uneasy - sail away" , "Is not well" , "Is unwell - sail away" .)