Nimble, mole round end of arbour (4)
I believe the answer is:
spry
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'mole round end of arbour' is the wordplay.
'mole' becomes 'spy' (I've seen this before).
'round' means one lot of letters goes inside another.
'end of' says to take the final letters.
The last letter of 'arbour' is 'r'.
'spy' enclosing 'r' is 'SPRY'.
(Other definitions for spry that I've seen before include "Fleet" , "Light on one's feet" , "Quick and agile" , "'Agile, nimble (4)'" , "Nimble (for one's age?)" .)