Harassed by fool who is troubled (8)
I believe the answer is:
assailed
'harassed' is the definition.
Although both the answer and definition are past participle verbs, I cannot see how they can define each other.
'fool who is troubled' is the wordplay.
I cannot really see how this works, but
'fool' could be 'ass' (both can mean a stupid person) and 'ass' is found in the answer.
'troubled' could be 'ailed' ('ail' can be a synonym of 'trouble') and 'ailed' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'by' acts as a link.
Can you help me to learn more?
(Other definitions for assailed that I've seen before include "charged" , "became aggressive" , "Violently attacked" , "Under attack" , "Set upon" .)