Try to cast youngster with webbed feet (7)
I believe the answer is:
gosling
'youngster with webbed feet' is the definition.
(a gosling is a young goose)
'try to cast' is the wordplay.
'try' becomes 'go' (go is a kind of try).
'to cast' becomes 'sling' (slinging is a kind of casting**).
'go'+'sling'='GOSLING'
(Other definitions for gosling that I've seen before include "Little bird" , "Young bird" , "Young goose" , "Young waterbird" .)