Badge mostly used by Mensa insiders and musicians (8)
I believe the answer is:
ensemble
'musicians' is the definition.
(I've seen this before)
'badge mostly used by mensa insiders' is the wordplay.
'badge' becomes 'emblem' (I've seen this in another clue).
'mostly' means to remove the last letter.
'used by' says to put letters next to each other.
'mensa insiders' becomes 'ens' (I can't explain this - if you can you should believe this answer much more).
'emblem' with its final letter removed is 'emble'.
'emble' after 'ens' is 'ENSEMBLE'.
'and' is the link.
Can you help me to learn more?
(Other definitions for ensemble that I've seen before include "Coordinated set of clothes" , "Company of players" , "Group of musicians or a co-ordinated outfit" , "Group of musicians playing together" , "Musical company" .)