Prevail upon journalist, a US agent, to turn (8)
I believe the answer is:
persuade
'prevail upon' is the definition.
(thesaurus)
'journalist a us agent to turn' is the wordplay.
'journalist' becomes 'ed' (short for editor).
'agent' becomes 'rep' (short for representative).
'to turn' says the letters should be written backwards.
'ed'+'a'+'us'+'rep'='edausrep'
'edausrep' in reverse letter order is 'PERSUADE'.
(Other definitions for persuade that I've seen before include "Cause (someone) to believe" , "Convince or induce, maybe coax" , "Convince by argument" , "Convince. cajole" , "Urge successfully" .)