Coach twenty regularly, or a somewhat smaller group (5)
I believe the answer is:
octet
'a somewhat smaller group' is the definition.
'octet' can be an answer for 'group' (group of eight musicians). I'm not certain of the 'a somewhat smaller' bit.
'coach twenty regularly' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'coach'+'twenty'='coachtwenty'
The alternate letters of 'coachtwenty' are 'OCTET'.
'or' is the link.
Can you help me to learn more?
(Other definitions for octet that I've seen before include "Music for eight" , "Composition for eight musicians" , "Piece for eight players" , "Mendelssohn or Schubert piece" , "What takes members up to eighth" .)