Suffers (4)
I believe the answer is:
ails
'suffers' is the definition.
(ailing is a kind of suffering)
This is the entire clue.
(Other definitions for ails that I've seen before include "Troubles; afflicts" , "Is unwell - sail away" , "Is not well" , "Is ill or uneasy - sail away" , "Feels unwell" .)