Composer keeping time in group (5)
I believe the answer is:
batch
'group' is the definition.
(a batch is a group or set)
'composer keeping time' is the wordplay.
'composer' becomes 'Bach' (Johann Sebastian Bach).
'keeping' means one lot of letters goes inside another.
'time' becomes 't' (abbreviation).
'bach' enclosing 't' is 'BATCH'.
'in' is the link.
(Other definitions for batch that I've seen before include "Group of similar people" , "Type of loaf" , "Lot made at one time" , "Production quantity" , "Collection of items handled together" .)