Sort of square rings on top of that (3)
I believe the answer is:
too
'on top of that' is the definition.
(I've seen this before)
'sort of square rings' is the wordplay.
'sort of square' becomes 't' (a T-Square is a technical drawing instrument).
'rings' becomes 'oo' (multiple circles).
't'+'oo'='TOO'
(Other definitions for too that I've seen before include "What a pity!" , "On top - also" , "As well; in addition" , "Excessively" , "Unduly" .)