Square mat used for basket (4)
I believe the answer is:
trug
'basket' is the definition.
(I've seen this before)
'square mat' is the wordplay.
'square' becomes 't' (a T-Square is a technical drawing instrument).
'mat' becomes 'rug' (I've seen this before).
't'+'rug'='TRUG'
'used for' is the link.
(Other definitions for trug that I've seen before include "Basket to use in 17" , "Fruit-basket" , "Small boat" , "Shallow basket for flowers, cuttings etc." , "Shallow oblong wooden basket" .)