In very small piece, it's certainly not square (4,2)
I believe the answer is:
with it
'certainly not square' is the definition.
'in very small piece it's' is the wordplay.
'in' means one lot of letters goes inside another.
'very small piece' becomes 'whit' (whit can mean a very small amount of something).
'whit' enclosing 'it' is 'WITH-IT'.
(Other definitions for with it that I've seen before include "At the cutting-edge" , "way gin might be served?" , "Progressive" , "Up-to-date" , "Trendy; in addition" .)