Stupid, like one joining a number (7)

I believe the answer is:

asinine

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

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

'like one joining a number' is the wordplay.
'like' becomes 'asi' (I can't explain this - if you can you should believe this answer much more).
'one joining' means one lot of letters go next to another.
'a number' becomes 'nine' (nine is a number).
'asi'+'nine'='ASININE'

Can you help me to learn more?

(Other definitions for asinine that I've seen before include "'Mindless, vacuous (7)'" , "Foolish like donkey" , "Idiotic" , "Extremely stupid or foolish" , "Absurd" .)

I've seen this clue in The Daily Express.

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.