The mount backed before (4)
I believe the answer is:
ante
'before' is the definition.
(prefix meaning before)
'the mount backed' is the wordplay.
'the mount' becomes 'Etna' (Etna is an example).
'backed' is a reversal indicator.
'etna' back-to-front is 'ANTE'.
(Other definitions for ante that I've seen before include "Stake, bet" , "Preceding birth" , "Advance stake" , "Betting stake" , "early payment" .)