If wrong again, dismiss! (4)
I believe the answer is:
fire
'dismiss' is the definition.
(I know that dismiss can be written as fire)
'if wrong again' is the wordplay.
'wrong' is an anagram indicator.
'again' becomes 're' (prefix meaning 'again').
'if' anagrammed gives 'fi'.
'fi'+'re'='FIRE'
(Other definitions for fire that I've seen before include "Let off" , "Shoot; sack" , "Ready, aim . . . ." , "There's no smoke without it" , "Inspire" .)