Burning a mark on a horse (6)
I believe the answer is:
ablaze
'burning' is the definition.
(I've seen this before)
'a mark on a horse' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
The remaining letters 'blaze' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for ablaze that I've seen before include "Having been fired" , "On fire, burning" , "being toasted?" , "Fiery" , "Well-lit" .)