Feel off (3)
I believe the answer is:
ail
'feel off' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ail that I've seen before include "Become sick" , "Be unwell, suffer" , "Sicken" , "Trouble, afflict; be sickly" , "Afflict -- feel unwell" .)