The fuss that's made when you open a door! (3)
I believe the answer is:
ado
'the fuss' is the definition.
('ado' can be a synonym of 'fuss')
'made when you open a door' is the wordplay.
I cannot quite see how this works, but
'a' is found in the answer.
The remaining letters 'do' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'that's' is the link.
Can you help me to learn more?
(Other definitions for ado that I've seen before include "Hoo-ha" , "Fuss, trouble" , "Flap" , "Rapid bustling commotion" , "Pother" .)