Not half mocked in jockey’s equipment? Leave fast! (9)
I believe the answer is:
skedaddle
'leave fast' is the definition.
(skedaddle can mean to run away)
'not half mocked in jockey's equipment?' is the wordplay.
'not half' means to take half the letters.
'in' indicates putting letters inside.
'jockey's equipment?' becomes 'saddle'.
'mocked' halved is 'ked'.
'ked' put inside 'saddle' is 'SKEDADDLE'.
(Other definitions for skedaddle that I've seen before include "Bunk off quickly!" , "Run off hurriedly (colloq.)" , "bolt" , "Scarper" , "split" .)