Jockey cleared the barrier finally (5)
I believe the answer is:
'jockey' is the definition.
(I've seen this before)
'cleared the barrier finally' is the wordplay.
'cleared' becomes 'rid' (as in ridding or clearing somewhere of something).
'finally' suggests the final letters.
The last letters of 'the barrier' are 'er'.
(Other definitions for rider that I've seen before include "Person on horse" , "Contract clause" , "Jury's recommendation" , "Extra provision" , "Possibly cavalier" .)