Hill has to start racing (3)
I believe the answer is:
tor
'hill' is the definition.
(tor is a kind of hill)
'to start racing' is the wordplay.
'start' says to take the initial letters.
The initial letter of 'racing' is 'r'.
'to'+'r'='TOR'
'has' acts as a link.
(Other definitions for tor that I've seen before include "Rocky outcrop" , "Hill or rocky peak" , "Dartmoor peak" , "Rising" , "It rises" .)