Leave supporter for one expected to rise to great heights (7)
I believe the answer is:
gosling
'one expected to rise to great heights' is the definition.
(a young goose which might one day fly high)
'leave supporter' is the wordplay.
'leave' becomes 'go'.
'supporter' becomes 'sling' (a sling supports an injured arm).
'go'+'sling'='GOSLING'
'for' is the link.
(Other definitions for gosling that I've seen before include "Little bird" , "Young goose" , "Young waterbird" , "Young bird" .)