Collection including extinct group (6)
I believe the answer is:
sextet
'collection' is the definition.
(sextet is a kind of collection)
'including extinct group' is the wordplay.
'including' indicates putting letters inside.
'extinct' becomes 'ext' (I can't justify this - if you can you should give a lot more credence to this answer).
'group' becomes 'set' (both can mean a collection).
'ext' inserted inside 'set' is 'SEXTET'.
Can you help me to learn more?
(Other definitions for sextet that I've seen before include "Chamber group (1)" , "Group of six (X, not S)" , "band" , "Group of six players" , "Piece for six" .)