How to tell it's playing well? (6)
I believe the answer is:
inform
'how to' is the definition.
I can't judge whether this definition defines the answer.
'tell it's playing well?' is the wordplay.
'tell' becomes 'in' (I can't justify this - if you can you should believe this answer much more).
'it's playing well?' becomes 'form' (as in a sportsperson who is playing 'on form').
'in'+'form'='INFORM'
Can you help me to learn more?
(Other definitions for inform that I've seen before include "Apprise: grass!" , "Let people know" , "Give knowledge to" , "Tell - shop" , "snitch?" .)