I'm still learning my bike, and I'm not an authority on this, but I have owned several Triumphs over the years and they all cluck to some extent when shifting into first gear. More so when they are cold.
You mentioned a shudder which I do not get on my bike a shudder and cluck are two different things.
Dinqua and Bennybmn are the benchmark when it comes to tech questions, you can take their advise to the bank.
Good luck and good riding.
