Jewel thief to hold his horses? (10)
I believe the answer is:
bloodstock
'his horses?' is the definition.
(bloodstock refers to horses bred for racing)
'jewel thief to hold' is the wordplay.
'jewel thief' becomes 'blood' (17th-century jewel thief Thomas Blood).
'to' means one lot of letters go next to another.
'hold' becomes 'stock' (hold can mean to stock or contain).
'blood'+'stock'='BLOODSTOCK'
(Other definitions for bloodstock that I've seen before include "Stable supply source" , "pedigree horses" , "Thoroughbreds" .)