Former pupil to yearn to get into shape (6)
I believe the answer is:
oblong
'shape' is the definition.
(oblong is a kind of shape)
'former pupil to yearn' is the wordplay.
'former pupil' becomes 'OB' (abbreviation for 'Old Boy' - private school pupil).
'to yearn' becomes 'long' ('long' can be a synonym of 'yearn').
'ob'+'long'='OBLONG'
'to get into' acts as a link.
(Other definitions for oblong that I've seen before include "Elongated rectangle" , "Oval" , "book form?" , "Roughly rectangular" , "Strip for planes" .)