Mental agility from a nimble but underweight monarch (5,8)
I believe the answer is:
quick thinking
'mental agility' is the definition.
(I've seen this in another clue)
'a nimble but underweight monarch' is the wordplay.
I cannot quite see how this works, but
'nimble' could be 'quick' (synonyms) and 'quick' is found in the answer.
'underweight' could be 'thin' (similar in meaning) and 'thin' is located in the answer.
'monarch' could be 'king' (king is a kind of monarch) and 'king' is located in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
'from' acts as a link.
Can you help me to learn more?
(Another definition for quick thinking that I've seen is " Presence of mind".)