Initially, old boys tease us, spoiling everything; a bit stupid (6)
I believe the answer is:
obtuse
'stupid' is the definition.
(similar in meaning)
'initially old boys tease us spoiling everything a bit' is the wordplay.
'initially' indicates taking the first letters.
'a bit' suggests taking the first letters (I've seen 'bit of' mean this).
The initial letters of 'old boys tease' is 'obt'.
The initial letters of 'us spoiling everything' is 'use'.
'obt'+'use'='OBTUSE'
(Other definitions for obtuse that I've seen before include "Slow of understanding" , "Thick, blunt" , "A stupid angle?" , "Blunt - stupid - insensitive" , "Angle greater than 90 degrees" .)