Romeo has left Harry for Mark (5)
I believe the answer is:
badge
'mark' is the definition.
(badging is a kind of marking)
'romeo has left harry' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'has left' is a deletion indicator.
'harry' becomes 'badger' (both can mean to bother or harass).
'badger' with 'r' removed is 'BADGE'.
'for' is the link.
(Other definitions for badge that I've seen before include "Thing worn to show rank or membership" , "Label with name or status" , "Distinguishing emblem" , "Worn sign of membership or support" , "Thing worn to show, say, membership of organisation" .)