You can put it on a horse that's a sure thing (5)
I believe the answer is:
cinch
'a sure thing' is the definition.
(I've seen this in another clue)
'you can put it on a horse' is the wordplay.
I cannot really understand how this works, but
'horse' could be 'h' (synonyms) and 'h' is found within the answer.
The remaining letters 'cinc' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for cinch that I've seen before include "simple job" , "Something easily done" , "A walk in the park" , "In slang, a certainty or easy task" , "Doddle" .)