Cart overturned on slope where knights once jousted (8)
I believe the answer is:
tiltyard
'where knights once jousted' is the definition.
'cart overturned on slope' is the wordplay.
'cart' becomes 'dray' (a cart without sides for delivering heavy loads).
'overturned' is a reversal indicator.
'on' means one lot of letters go next to another.
'slope' becomes 'tilt' (I've seen this before).
'dray' in reverse letter order is 'yard'.
'yard' put after 'tilt' is 'TILTYARD'.
(Other definitions for tiltyard that I've seen before include "where Henry suffered wounds" , "Enclosed area for jousting" .)