Mounting horse with regulation step gets run over (5)
I believe the answer is:
recap
'run over' is the definition.
(I've seen this before)
'mounting horse with regulation step' is the wordplay.
'mounting' is a reversal indicator (going up in a down clue).
'horse with regulation step' becomes 'pacer' (horse which paces).
'pacer' reversed gives 'RECAP'.
'gets' is the link.
(Other definitions for recap that I've seen before include "Summarise briefly (abbr)" , "play again" , "Caper (anag)" , "Quickly run through" , "sum everything up" .)