Replacement bass put in an appearance in auditorium (5)
I believe the answer is:
locum
'replacement' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for locum that I've seen before include "Doctor's stand-in" , "Temporary replacement doctor" , "may step into someone else's job" , "Stand-in, for a doctor say" , "Person filling in" .)