It’s hard to cut the string (6)
I believe the answer is:
severe
'it's hard to' is the definition.
(I've seen this before)
'cut the string' is the wordplay.
'cut' becomes 'sever' (to sever is to cut or split).
'the string' becomes 'e' (I can't explain this - if you can you should give a lot more credence to this answer).
'sever'+'e'='SEVERE'
Can you help me to learn more?
(Other definitions for severe that I've seen before include "Extreme - strict" , "Strict and unbending" , "Harsh and stern" , "Forbidding in action or appearance" , "Austere, spartan" .)