Musical replacing opening with another key line (6)
I believe the answer is:
crease
'line' is the definition.
(synonyms)
'musical replacing opening with another key' is the wordplay.
I cannot quite see how this works, but
'another' could be 're' (I've seen this in other clues) and 're' is found in the answer.
'key' could be 'e' (a musical key) and 'e' is found within the answer.
The remaining letters 'cas' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for crease that I've seen before include "Folded line" , "Fold in clothes" , "Line made by folding" , "Furrow, wrinkle" , "Dagger rarely" .)