Guess users enraged about M1 (7)
I believe the answer is:
surmise
'guess' is the definition.
(I know that surmise is a type of guess)
'users enraged about m1' is the wordplay.
'enraged' indicates anagramming the letters.
'about' is an insertion indicator.
'm1' becomes 'mi' (resembles M1).
'users' anagrammed gives 'surse'.
'surse' enclosing 'mi' is 'SURMISE'.
(Other definitions for surmise that I've seen before include "Hazard a guess" , "Make a conjecture" , "Speculate about misuser" , "Guess, conjecture" , "Presume -- rue miss (anag)" .)