Mount you put a bet on, when backing (4)
I believe the answer is:
etna
'mount you' is the definition.
'etna' can be an answer for 'mount' (I've seen this before). I'm not sure about the 'you' bit.
'a bet on when backing' is the wordplay.
'a bet' becomes 'eta' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' means one lot of letters goes inside another (as in clothing \'on\' a person).
'backing' suggests the final letters.
The last letter of 'when' is 'n'.
'eta' placed around 'n' is 'ETNA'.
'put' is the link.
Can you help me to learn more?
(Other definitions for etna that I've seen before include "source of fumes?" , "Highest active volcano in Europe" , "Explosive type" , "Sicilian hothead?" , "Volcano in Sicily, quite neat" .)