Rich performance includes old singers (5)
I believe the answer is:
choir
'singers' is the definition.
(group of singers)
'rich performance includes old' is the wordplay.
'performance' is an anagram indicator.
'includes' means one lot of letters goes inside another.
'old' becomes 'o' (abbreviation).
'rich' is an anagram of 'chir'.
'chir' enclosing 'o' is 'CHOIR'.
(Other definitions for choir that I've seen before include "Large group of singers" , "set of singers" , "Body of singers" , "Carol singers?" , "Religious chorus, perhaps" .)