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)
'being good in a group' is the wordplay.
'being good' becomes 'ok'.
'in' indicates putting letters inside.
'a group' becomes 'ten' (I am not sure about this - if you are sure you should believe this answer much more).
'ok' inserted inside 'ten' is 'TOKEN'.
'of' is the 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" .)