Registered in excessively quiet group of scouts (5)
I believe the answer is:
troop
'group of scouts' is the definition.
(I've seen this in another clue)
'registered in excessively quiet' is the wordplay.
'registered' becomes 'R' (abbreviation, as in registered trademark).
'in' means one lot of letters goes inside another.
'excessively' becomes 'too' (to too great a degree).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'r' put within 'too' is 'troo'.
'troo'+'p'='TROOP'
(Other definitions for troop that I've seen before include "Military body" , "Cavalry unit commanded by a captain" , "Body of soldiers or scouts" , "herd" , "Flock, large group" .)