Leave to throw a young bird (7)
I believe the answer is:
gosling
'a young bird' is the definition.
(I've seen this before)
'leave to throw' is the wordplay.
'leave' becomes 'go' (to go is to depart or leave).
'to throw' becomes 'sling' (chuck means throw as does sling).
'go'+'sling'='GOSLING'
(Other definitions for gosling that I've seen before include "Young bird" , "Young goose" , "Little bird" , "Young waterbird" .)