Two extras to add to that (11)

I believe the answer is:

furthermore

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

'to add to that' is the definition.
('furthermore' is said to add something else)

'two extras' is the wordplay.
'two extras' can mean two substitutions for 'extra'.
'extra' becomes 'further' (I can't explain this - if you can you should give a lot more credence to this answer).
'extra' becomes 'more' (both can mean additional).
'further'+'more'='FURTHERMORE'

Can you help me to learn more?

(Other definitions for furthermore that I've seen before include "as well" , "In addition, besides" , "to boot" , "Also" .)

I've seen this clue in The Times.

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.