Let it be one sort of square in another (4)
I believe the answer is:
stet
'let it be' is the definition.
(printing instruction to let a piece of text stand)
'one sort of square in another' is the wordplay.
'one sort of square' becomes 'T' (a T-Square is a technical drawing instrument).
'in' indicates putting letters inside.
'another' becomes 'set' (I can't explain this - if you can you should believe this answer much more).
't' placed into 'set' is 'STET'.
Can you help me to learn more?
(Other definitions for stet that I've seen before include "Don't alter, let spelling stand" , "Cancellation of correction" , "Let it stand - printer's mark" , "Leave as is - don't amend" , "'In printing, cancel a correction and leave it as it was (4)'" .)