Some dealer terribly vigilant (5)
I believe the answer is:
alert
'vigilant' is the definition.
(similar in meaning)
'some dealer terribly' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'ALERT' is hidden in the letters of 'dealer terribly'.
(Other definitions for alert that I've seen before include "Wide-awake and watchful" , "In state of watchfulness" , "Watchful; warning" , "Watchful and bright" , "'Attentive, vigilant (5)'" .)