Drifter was harassed by FBI agent (7)
I believe the answer is:
swagman
'drifter' is the definition.
(I've seen this before)
'was harassed by fbi agent' is the wordplay.
'harassed' indicates anagramming the letters.
'by' says to put letters next to each other.
'fbi agent' becomes 'G-man' (synonyms).
'was' with letters rearranged gives 'swa'.
'swa'+'gman'='SWAGMAN'
(Other definitions for swagman that I've seen before include "Aussie worker" , "itinerary" , "Drifter" , "Australian tramp" .)