Fairly ugly jockey is starting to mount (4)
I believe the answer is:
fuji
'to mount' is the definition.
(I have seen 'Mount of Japan' mean 'fuji' so perhaps 'mount' could also mean 'fuji')
'fairly ugly jockey is starting' is the wordplay.
'starting' suggests taking the first letters.
The initial letters of 'fairly ugly jockey is' is 'FUJI'.
(Other definitions for fuji that I've seen before include "Sacred Japanese mountain" , "Extinct volcano in central Japan" , "Japanese mount" , "Extinct Japanese volcano - cherry tree" , "Japan's highest peak" .)