Tenacious, if troubled (6)
I believe the answer is:
dogged
I believe this clue is a double definition.
'tenacious' is the first definition.
(I know that tenacious can be written as dogged)
'troubled' is the second definition.
(dog can mean to follow or torment)
'if' acts as a link.
(Other definitions for dogged that I've seen before include "Shadowed; persistent" , "Stubborn, in a canine kind of way" , "Grimly persistent" , "Tenacious, persistent" , "Relentless in a canine way" .)