Ultra (7)
'ultra' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for extreme that I've seen before include "Ultimate" , "Going to great lengths" , "end" , "Anything but moderate" , "Very great; exceptional" .)
(Other definitions for extremely that I've seen before include "remarkably" , "To a high degree or extent" .)