Teacher initially trying to stop retreating rabble (5)
I believe the answer is:
tutor
'teacher' is the definition.
(tutor is a kind of teacher)
'initially trying to stop retreating rabble' is the wordplay.
'initially' says to take the initial letters.
'to stop' indicates putting letters inside.
'retreating' says the letters should be written backwards.
'rabble' becomes 'rout' ('rout' can be a synonym of 'rabble').
The first letter of 'trying' is 't'.
'rout' backwards is 'tuor'.
't' put into 'tuor' is 'TUTOR'.
(Other definitions for tutor that I've seen before include "Private educator" , "Give individual instruction" , "college teacher" , "Personal teacher" , "Master" .)