Stupid in the way that number must cross island (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'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'.
'as'+'nine'='asnine'
'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" .)