Corner is not to be cut, you understand? (4)
I believe the answer is:
nook
'corner' is the definition.
(synonyms)
'not to be cut you understand?' is the wordplay.
'to be cut' means to remove the last letter.
'you understand?' becomes 'ok' (I can't explain this - if you can you should believe this answer much more).
'not' with its final letter taken away is 'no'.
'no'+'ok'='NOOK'
'is' is the link.
Can you help me to learn more?
(Other definitions for nook that I've seen before include "Alcove" , "Interior angle formed where two walls meet" , "Secluded recess" , "Recess or cranny" , "Sheltered and secluded place" .)