Which club did declarer play at trick one? If it was the six, partner has played his lowest club ....
If pard has x AKxx Qxxx QJT9 it seems like he should help us out here. Natural signals generally take precedence over SP at trick one, so he should play the queen just to say clubs are under control. That will stop me playing a disastrous diamond.
If he has x AKxx QJT9x JT9, he can play the jack, telling me clubs are running and encouraging the cash out.
And if pard has x Axxx Kxxxx QJ9, where we need to switch to a diamond, then yes - he will play the nine by default. So that's what I will do. Edit this is wrong. Diamond switch is not necessary. I'm struggling for a rational hand (I can think of one where pard has a nine count) where a diamond switch is necessary to beat it, but it seems that's what partner wants.
All three scenarios indicate that declarer was having a laugh in the bidding, but that's how some people roll these days.
Scrap all of the aboe on the basis that OP was not correct.