Former pupil finds lengthy shape (6)
I believe the answer is:
oblong
'shape' is the definition.
(oblong is a kind of shape)
'former pupil finds lengthy' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'finds' means one lot of letters go next to another.
'lengthy' becomes 'long' (synonyms).
'ob'+'long'='OBLONG'
(Other definitions for oblong that I've seen before include "Rectangular two-dimensional figure" , "Oval" , "Figure with opposite sides parallel, longer in one direction" , "Rectangular but not square" , "Cab" .)