Group: into eighteen one twice goes! (5)
I believe the answer is:
nonet
'group' is the definition.
(group of nine musicians)
'into eighteen one twice goes' is the wordplay.
'into' indicates the answer is hidden within the clue.
'NONET' is hidden within 'eighteen one twice goes'.
(Other definitions for nonet that I've seen before include "Band's" , "Composition for nine performers" , "Band of nine?" , "Music for nine instruments or voices" , "Group of singers" .)