If not striking a threatening note? (6)
I believe the answer is:
unless
'if' is the definition.
(I've seen this before)
'not striking a threatening note?' is the wordplay.
I cannot really see how this works, but
'not' could be 'less' (I've seen this in other clues) and 'less' is located in the answer.
'note?' could be 'n' (abbreviation) and 'n' is found in the answer.
A single letter 'u' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for unless that I've seen before include "bar" , "Except, save" , "Conjunction meaning except when" , "Otherwise than if" , "Though" .)