Stupid, like idiot’s opening figure (7)
I believe the answer is:
asinine
'stupid' is the definition.
(thesaurus)
'like idiot's opening figure' is the wordplay.
'like' becomes 'as'.
'opening' indicates taking the first letters.
'figure' becomes 'nine' (I've seen this in other clues**).
The initial letter of 'idiots' is 'i'.
'as'+'i'+'nine'='ASININE'
(Other definitions for asinine that I've seen before include "Extremely stupid or foolish" , "'Mindless, vacuous (7)'" , "Is inane to be foolish" , "Foolish like donkey" , "Daft" .)