Journalist runs into alleged adulterer (13)
I believe the answer is:
correspondent
'journalist' is the definition.
(correspondent is a kind of journalist)
'runs into alleged adulterer' is the wordplay.
'runs' becomes 'r' (cricket abbreviation).
'into' means one lot of letters goes inside another.
'alleged adulterer' becomes 'co-respondent' (legal term from divorce cases).
'r' inserted inside 'corespondent' is 'CORRESPONDENT'.
(Other definitions for correspondent that I've seen before include "Overseas journalist" , "Newspaper writer" , "Journalist; pen pal" , "Pen-friend" , "Person sending letters or written reports - agreeing" .)