Attentive dealer taking part (5)
I believe the answer is:
alert
'attentive' is the definition.
(I've seen this before)
'dealer taking part' is the wordplay.
'part' indicates the answer is hidden within the clue.
'ALERT' can be found hidden inside 'dealer taking'.
(Other definitions for alert that I've seen before include "Watchful; warning" , "Intellectually active" , "alarm" , "Prompt" , "On the ball" .)