Singer in charge included rudiments (6)
I believe the answer is:
basics
'rudiments' is the definition.
(synonyms)
'singer in charge included' is the wordplay.
'singer' becomes 'bass' (bass is a kind of singer).
'in charge' becomes 'ic' (abbreviation for 'in charge').
'included' is an insertion indicator.
'bass' going around 'ic' is 'BASICS'.
(Other definitions for basics that I've seen before include "Fundamentals, starting-points" , "Essential principles" , "Rudiments" .)