Group — new one beginning to trend (5)
I believe the answer is:
nonet
'group' is the definition.
(group of nine musicians)
'new one beginning to trend' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'beginning to' says to take the initial letters.
The initial letter of 'trend' is 't'.
'n'+'one'+'t'='NONET'
(Other definitions for nonet that I've seen before include "Piece for nine musicians" , "group composition" , "baseball team" , "Music for nine instruments or voices" , "Work for nine players" .)