Stupid when one joins a larger number (7)

I believe the answer is:

asinine

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

'when one joins a larger number' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'one' becomes 'i' (Roman numeral).
'joins' means one lot of letters go next to another.
'a larger number' becomes 'nine' (nine is a number. I am not sure about the 'larger' bit.).
'as'+'i'+'nine'='ASININE'

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(Other definitions for asinine that I've seen before include "Foolish like donkey" , "Jenny's often like this" , "Senseless" , "'Mindless, vacuous (7)'" , "Idiotic" .)

I've seen this clue in The Daily Express.

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