Sh! I must tear off the narrow strip (7)
I believe the answer is:
isthmus
'strip' is the definition.
(I've seen this before)
'sh i must tear' is the wordplay.
'tear' indicates anagramming the letters.
'sh'+'i'+'must'='shimust'
'shimust' anagrammed gives 'ISTHMUS'.
'off the narrow' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for isthmus that I've seen before include "Strip of land flanked by sea" , "Narrow strip of land connecting two larger land masses" , "eg Corinth, Panama" , "Narrow strip of land with water on two sides" , "Narrow strip of land in sea" .)