In America, harbor is the setting for tree doctor (8)
I believe the answer is:
arborist
'tree doctor' is the definition.
(I know that tree doctor can be written as arborist)
'in america harbor is the' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ARBORIST' can be found hidden inside 'america harbor is the'.
'setting for' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for arborist that I've seen before include "tree surgeon" , "Tree expert" .)