Group including a precise number (6)
I believe the answer is:
sextet
'number' is the definition.
(sextet is a kind of whole number)
'group including a precise' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'including' is an insertion indicator.
'a precise' becomes 'ext' (I can't explain this - if you can you should believe this answer much more).
'set' enclosing 'ext' is 'SEXTET'.
(Other definitions for sextet that I've seen before include "Half-dozen in a group" , "Musical group" , "Group of six players" , "Group of six performers" , "Chamber group (1)" .)