Some sign of being good in a group (5)
I believe the answer is:
token
'some sign' is the definition.
(token is a kind of sign)
'good in a group' is the wordplay.
'good' becomes 'ok'.
'in' is an insertion indicator.
'a group' becomes 'ten' (I can't explain this - if you can you should give a lot more credence to this answer).
'ok' going into 'ten' is 'TOKEN'.
'of being' acts as a link.
Can you help me to learn more?
(Other definitions for token that I've seen before include "Done only as a symbolic gesture" , "Money substitute" , "Emblem" , "Sign, symbol or an evidence" , "Nominal; voucher" .)