Obstruction knocked over by a fast-moving horse (4)
I believe the answer is:
arab
'horse' is the definition.
(Arab is a type of horse)
'obstruction knocked over by a' is the wordplay.
'obstruction' becomes 'bar' (bar is a kind of obstruction).
'knocked over' says the letters should be written backwards.
'by' says to put letters next to each other.
'bar' in reverse letter order is 'rab'.
'rab' after 'a' is 'ARAB'.
'fast-moving' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for arab that I've seen before include "Person from the Middle East, say" , "Fine horse breed" , "Saudi, e.g." , "One from the Middle East, say" , "Saudi, perhaps - breed of horse" .)