Has a bug (4)
I believe the answer is:
ails
'has a bug' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ails that I've seen before include "Is unwell - sail away" , "Troubles; afflicts" , "Is ill or uneasy - sail away" , "Is not well" , "Is sick" .)