Jockey replaces these for a Grand National runner, say (13)
I believe the answer is:
steeplechaser
'a grand national runner say' is the definition.
(the Grand National is a steeplechase)
'jockey replaces these' is the wordplay.
'jockey' is an anagram indicator (the letters scramble or jockey around).
'replaces'+'these'='replacesthese'
'replacesthese' anagrammed gives 'STEEPLECHASER'.
'for' acts as a link.
(Other definitions for steeplechaser that I've seen before include "Competitive runner" , "Racehorse - athlete" , "Cross-country runner" , "Racehorse; 13" , "Horse in Grand National, for example" .)