Coal dust found in the middle of Ascot round all jumps (4)
I believe the answer is:
culm
'coal dust' is the definition.
(I've seen this before)
'middle of ascot round all jumps' is the wordplay.
'middle of' indicates the central letters.
'round all jumps' becomes 'ulm' (I can't justify this - if you can you should give a lot more credence to this answer).
The central letter of 'ascot' is 'c'.
'c'+'ulm'='CULM'
'found in the' is the link.
Can you help me to learn more?
(Other definitions for culm that I've seen before include "Coal-dust" , "coal dust" .)