I heard you are the guy letting fly extremes compelling immediate action (8)
I believe the answer is:
urgently
'i heard you are the guy letting fly extremes' is the definition.
I can't judge whether this defines the answer.
'compelling immediate action' is the wordplay.
'compelling' becomes 'urgent' (I've seen this before).
'immediate action' becomes 'ly' (I can't explain this - if you can you should believe this answer much more).
'urgent'+'ly'='URGENTLY'
Can you help me to learn more?
(Other definitions for urgently that I've seen before include "as quickly as possible?" , "without delay" , "Pressingly" , "With speedy attention" , "As a matter of priority" .)