Group of Scouts upsetting poor teacher at first (5)
I believe the answer is:
troop
'group of scouts' is the definition.
(I've seen this in another clue)
'upsetting poor teacher at first' is the wordplay.
'upsetting' is a reversal indicator.
'at first' indicates taking the first letters.
The first letter of 'teacher' is 't'.
'poor'+'t'='poort'
'poort' written backwards gives 'TROOP'.
(Other definitions for troop that I've seen before include "Cavalry unit commanded by a captain" , "Military unit" , "A company of people, soldiers or scouts" , "Flock; Scout group" , "Proto (anag.)" .)