Expensive contract to retain champion for horse race (12)
I believe the answer is:
steeplechase
'horse race' is the definition.
(steeplechase is a kind of horse race)
'expensive contract to retain champion' is the wordplay.
'expensive' becomes 'steep' (I've seen this before).
'contract' becomes 'lease' (leasing is a kind of contracting).
'to retain' is an insertion indicator.
'champion' becomes 'ch' (I've seen this in other clues).
'lease' placed around 'ch' is 'lechase'.
'steep'+'lechase'='STEEPLECHASE'
'for' is the link.
(Other definitions for steeplechase that I've seen before include "Non-flat horse race" , "Running event over hurdles and water jumps" , "Maybe Grand National" , "Race over fences" , "Horse race with ditches and hedges" .)