The mount backed before (4)
I believe the answer is:
ante
'before' is the definition.
(I've seen this in other clues)
'the mount backed' is the wordplay.
'the mount' becomes 'etna' (Etna is an example).
'backed' says the letters should be written in reverse.
'etna' backwards is 'ANTE'.
(Other definitions for ante that I've seen before include "Stake, bet" , "Preceding birth" , "Advance stake" , "Betting stake" , "early payment" .)