High-flier's dog with no lead (5)
I believe the answer is:
eagle
'high-flier's' is the definition.
(I've seen this before)
'dog with no lead' is the wordplay.
'dog' becomes 'beagle' (beagle is a kind of dog).
'with no lead' means to remove the first letter.
'beagle' with its initial letter taken off is 'EAGLE'.
(Other definitions for eagle that I've seen before include "high-flier" , "Erne, perhaps" , "Raptor" , "Keen-sighted bird of prey" , "High-flyer" .)