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 am not sure about this - if you are sure you should give a lot more credence to this answer).
'not' with its final letter removed 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" .)