Initially head off a sign of voting fraud (4)
I believe the answer is:
hoax
'fraud' is the definition.
(synonyms)
'initially head off a sign of voting' is the wordplay.
'initially' suggests taking the first letters.
'sign of voting' becomes 'X' (as in a cross on a ballot paper).
The initial letters of 'head off' is 'ho'.
'ho'+'a'+'x'='HOAX'
(Other definitions for hoax that I've seen before include "Leg-pull" , "Act of deception" , "Fake, such as Piltdown Man" , "Spoof" , "Prank" .)