What one's prompted to do on a psychiatrist's couch (4,2,6)

'a psychiatrist's couch' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'what one's prompted to do on' is the wordplay.
I cannot really see how this works, but
'on' could be 'at' (I've seen this in other clues) and 'at' is located in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?