Is backed in horse racing with caution (4)
I believe the answer is:
care
'caution' is the definition.
('care' can be a synonym of 'caution')
'is backed in horse racing' is the wordplay.
'is backed' says the letters should be written backwards.
'in' indicates the answer is hidden within the clue.
'erac' is hidden within 'horse racing'.
'erac' backwards is 'CARE'.
'with' is the link.
(Other definitions for care that I've seen before include "Be concerned or nurse" , "Consideration" , "Safekeeping" , "Caution; keeping" , "Have affection (for)" .)