Time to employ former pupil before getting dull-witted (6)
I believe the answer is:
obtuse
'dull-witted' is the definition.
(I've seen this in another clue)
'time to employ former pupil before' is the wordplay.
'time' becomes 't' (abbreviation).
'to employ' becomes 'use' ('use' can be a synonym of 'employ').
'former pupil' becomes 'OB' (abbreviation for 'Old Boy').
'before' means one lot of letters go next to another.
't'+'use'='tuse'
'tuse' after 'ob' is 'OBTUSE'.
'getting' is the link.
(Other definitions for obtuse that I've seen before include "Blunt, not sensitive" , "angle, perhaps" , "(Of angle) between 90deg and 180deg - dull" , "Blunt, not sharp or pointed" , "Angle between 90 and 180 degrees" .)