Caress most of bottom frequently to begin with (one's amenable) (1,4,5)

'one's amenable' is the definition.

'caress most of bottom frequently to begin' is the wordplay.
I cannot really see how this works, but
'caress' could be 'softtouch' (I've seen this before) and 'softtouch' is found in the answer.
A single letter 'a' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...

'with' acts as a link.