Warning saying that the diamonds are fake? (6)
I believe the answer is:
notice
'warning saying that the diamonds are fake?' is the definition.
'notice' can be an answer for 'warning' (I've seen this in another clue). I am unsure of the remainder of the definition.
'warning saying that the diamonds' is the wordplay.
'warning saying' becomes 'not' (I can't explain this - if you can you should give a lot more credence to this answer).
'that the diamonds' becomes 'ice' (slang term for diamonds).
'not'+'ice'='NOTICE'
Can you help me to learn more?
(Other definitions for notice that I've seen before include "warning to quit" , "Advance warning of job resignation" , "Notification" , "Instruction" , "Poster; observe" .)