Arboreal monarch (3)
I believe the answer is:
oak
'arboreal monarch' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'arboreal monarch' is the wordplay.
'arboreal' becomes 'oa' (I can't explain this - if you can you should give a lot more credence to this answer).
'monarch' becomes 'K' (chess abbreviation).
'oa'+'k'='OAK'
Can you help me to learn more?
(Other definitions for oak that I've seen before include "The mighty tree" , "Grown acorn" , "Hardwood tree" , "Common forest tree" , "Hard durable wood of a deciduous tree" .)