Excellent damson (4)
I believe the answer is:
plum
'damson' is the definition.
(thesaurus)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for plum that I've seen before include "Dark reddish-purple colour: especially valued" , "Brilliant" , "Fruit tree - of especial value?" , "First-class - fruit" , "Very desirable thing" .)