One to control dog in the French wood (5)
I believe the answer is:
'one to control' is the definition.
(I've seen this before)
'the french wood' is the wordplay.
'the french' becomes 'le' ('the' in French).
'wood' becomes 'ash' (wood from an ash tree).
'dog in' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for leash that I've seen before include "Restraining strap for dog" , "(Dog's) lead" , "Line for controlling a dog" , "Restraint for a dog" , "Rope or chain restraint" .)