... transported, opening door firstly (9)

I believe the answer is:

entranced

I'm a little stuck... Click here to teach me more about this clue!

'transported' is the definition.
(I've seen this before)

'opening door firstly' is the wordplay.
'opening' becomes 'entrance' (opening leading to a building).
'door firstly' becomes 'd' (I can't justify this - if you can you should give a lot more credence to this answer).
'entrance'+'d'='ENTRANCED'

Can you help me to learn more?

(Other definitions for entranced that I've seen before include ""Spellbound"?" , "Fascinated" , "Captivated" , "Filled with wonder" , "Delighted" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.