Warning dealer to take part (5)
I believe the answer is:
alert
'warning' is the definition.
(I know that alert is a type of warning)
'dealer to take part' is the wordplay.
'take part' indicates a hidden word (I've seen 'taking part of' mean this).
'ALERT' is hidden in the letters of 'dealer to'.
(Other definitions for alert that I've seen before include "Watchful, sharp" , "Vigilant, watchful" , "Watchful, quick-thinking" , "Lively" , "Signal danger" .)