Declare a vicar has turned up (4)
I believe the answer is:
aver
'declare' is the definition.
(averring is a kind of declaring)
'a vicar has turned up' is the wordplay.
'vicar' becomes 'rev'.
'has turned up' is a reversal indicator.
'rev' in reverse letter order is 'ver'.
'a'+'ver'='AVER'
(Other definitions for aver that I've seen before include "Assert, state as true" , "Formally assert" , "State firmly and positively" , "State; say" , "State firmly - don't rave" .)