Urgent message - false gem alert (8)
I believe the answer is:
telegram
'urgent message' is the definition.
'telegram' can be an answer for 'message' (telegram is a kind of message). I am not certain of the 'urgent' bit.
'false gem alert' is the wordplay.
'false' is an anagram indicator.
'gem'+'alert'='gemalert'
'gemalert' with letters rearranged gives 'TELEGRAM'.
Can you help me to learn more?
(Other definitions for telegram that I've seen before include "Message transmitted by wire" , "Now obsolete communication over a wire" , "Message sent by wire" , "cable" , "Dispatch" .)