Journalist on carrier (8)
I believe the answer is:
reporter
'journalist' is the definition.
(eg a reporter for a newspaper)
'on carrier' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'carrier' becomes 'porter'.
're'+'porter'='REPORTER'
(Other definitions for reporter that I've seen before include "Journo" , "One writing in newspaper" , "Newspaper writer" , "Newsperson" , "One writes or tells the news" .)