Increasing when one's getting into the saddle (8,2)

'increasing' is the definition.
Although both the answer and definition are present participle verbs, I can't see how one could define the other.

'one's getting into the saddle' is the wordplay.
I cannot quite understand how this works, but
'into' could be 'in' and 'in' is located in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'one' could be 'un' (northern English dialect form of 'one') and 'un' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'when' is the link.

Can you help me to learn more?