Maybe one astride a heavy carthorse? (9)
I believe the answer is:
percheron
'carthorse?' is the definition.
(I know that Percheron is a type of carthorse)
'maybe one astride a heavy' is the wordplay.
I cannot quite see how this works, but
'a' could be 'per' (eg 'one a day' means 'one per day') and 'per' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for percheron that I've seen before include "heavy horse" , "Carthorse" , "draught horse" .)