Old boy has itch to be 'not square' (6)
I believe the answer is:
oblong
'not square' is the definition.
'old boy has itch' is the wordplay.
'old boy' becomes 'ob'.
'has' is a charade indicator (letters next to each other).
'itch' becomes 'long'.
'ob'+'long'='OBLONG'
'to be' acts as a link.
(Other definitions for oblong that I've seen before include "book form?" , "Rectangle with adjacent sides unequal" , "Log on B (anag.)" , "Cab" , "Roughly rectangular" .)