Stupid in the way that number must cross island (7)
I believe the answer is:
'stupid' is the definition.
'in the way that number must cross island' is the wordplay.
'in the way that' becomes 'as'.
'number' becomes 'nine' (nine is a kind of number).
'must cross' means one lot of letters goes inside another.
'island' becomes 'i'.
'asnine' placed around 'i' is 'ASININE'.
(Other definitions for asinine that I've seen before include "Is Annie? (Anag)" , "In the nature of supposedly stupid animal" , "Foolish like donkey" , "Really silly, ridiculous" , "Jenny's often like this" .)