Gossip once grand slam broadcast (13)
I believe the answer is:
scandalmonger
'gossip' is the definition.
(I've seen this before)
'once grand slam broadcast' is the wordplay.
'broadcast' is an anagram indicator.
'once'+'grand'+'slam'='oncegrandslam'
'oncegrandslam' with letters rearranged gives 'SCANDALMONGER'.
(Other definitions for scandalmonger that I've seen before include "Destroyer of reputation" , "Spreader of malicious gossip" , "Rumour spreader" , "Tattler" , "Person spreading malicious tittle-tattle" .)