Jumps into the saddle and sets off (6)

'sets off?' is the definition.
'mounts' can be an answer for 'sets' (mounting is a kind of setting). I am not certain of the 'off?' bit.

'jumps into the saddle and' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
The remaining letters 'mous' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?