Former pupil finds the first application stupid (6)
I believe the answer is:
obtuse
'stupid' is the definition.
(similar in meaning)
'former pupil finds the first application' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'finds' says to put letters next to each other.
'the first' becomes 't' (1st letter of 'the').
'application' becomes 'use' (I've seen this before).
'ob'+'t'+'use'='OBTUSE'
(Other definitions for obtuse that I've seen before include "boneheaded?" , "Blunt, not sensitive" , "Dull, non-perceptive" , "To be thus blunt" , "(Of angle) between 90deg and 180deg - dull" .)