Maybe one astride a heavy carthorse? (9)
I believe the answer is:
percheron
'carthorse?' is the definition.
(I've seen this before)
'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 found within 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" .)