Alumnus, boring perhaps, certainly not square (6)
I believe the answer is:
oblong
'certainly not square' is the definition.
'alumnus boring perhaps' is the wordplay.
'alumnus' becomes 'ob' (old boy).
'boring perhaps' becomes 'long'.
'ob'+'long'='OBLONG'
(Other definitions for oblong that I've seen before include "Rectangle with adjacent sides unequal" , "Cab" , "tablet format?" , "Rectangular but not square" , "Oval" .)