Brief -- unless before end of play (6)
I believe the answer is:
notify
'brief' is the definition.
(thesaurus)
'unless before end of play' is the wordplay.
'unless' becomes 'noti' (I am not sure about this - if you are sure you should believe this answer much more).
'before' is a charade indicator (letters next to each other).
'end' indicates one should take the final letters.
The final letters of 'of play' are 'fy'.
'noti'+'fy'='NOTIFY'
Can you help me to learn more?
(Other definitions for notify that I've seen before include "'Advise, tell about (6)'" , "Formally inform" , "Formally advise" , "alert" , "Officially inform" .)