Leading man's new bird (5)
I believe the answer is:
heron
'bird' is the definition.
(heron is a kind of bird)
'leading man's new' is the wordplay.
'leading' means one lot of letters go next to another.
'man' becomes 'hero' (I have seen 'Courageous man' mean 'hero' so perhaps 'man' could also mean 'hero').
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'hero' put next to 'n' is 'HERON'.
(Other definitions for heron that I've seen before include "Bird of the Rhone, strangely" , "Big bird" , "One bird" , "Graceful grey bird with long neck" , "Tall wading bird" .)