Vigilant fellow finally gets a fluke break (7)
I believe the answer is:
wakeful
'vigilant' is the definition.
(I know that vigilant can be written as wakeful)
'fellow finally gets a fluke break' is the wordplay.
'finally' suggests the final letters.
'gets' says to put letters next to each other.
'break' indicates anagramming the letters.
The last letter of 'fellow' is 'w'.
'fluke' with letters rearranged gives 'keful'.
'w'+'a'+'keful'='WAKEFUL'
(Other definitions for wakeful that I've seen before include "Not sleepy" , "Not getting off" , "Unable to sleep" , "Alert, vigilant" .)