A certain pace produced by the position of a jockey (7)
I believe the answer is:
astride
'a certain pace produced' is the definition.
I can't tell whether this definition defines the answer.
'by the position of a jockey' is the wordplay.
'by' says to put letters next to each other.
'the position of' becomes 'ast' (I can't justify this - if you can you should give a lot more credence to this answer).
'a jockey' becomes 'ride' (I've seen this in another clue).
'ast' next to 'ride' is 'ASTRIDE'.
Can you help me to learn more?
(Other definitions for astride that I've seen before include "With one leg on either side, as sitting on horse" , "On the back of" , "Straddling (horse)" , "Not side saddle" , "It dares to be with legs wider apart" .)