Nimble, mole round end of arbour (4)
I believe the answer is:
Here is my best explanation:
'nimble' is the definition.
('spry' can be a synonym of 'nimble')
'mole round end of arbour' is the subsidiary indication.
'mole' becomes 'spy' (I've seen this before).
'round' means one lot of letters goes inside another.
'end of' indicates one should take the final letters.
The last letter of 'arbour' is 'r'.
'spy' placed around 'r' is 'spry'.
(Other definitions for spry that I've seen before include "Nimble, agile", "Light on one's feet", "Sprightly", "(Of an old person) lively", "Far from doddering", "Nimble and lively", "Alert", "Agile (for one's age?)", "Moving quickly and lightly" and "Nimble, brisk".)