Dirt on fellow worker (7)
I believe the answer is:
dustman
'worker' is the definition.
(dustman is a kind of worker)
'dirt on fellow' is the wordplay.
'dirt' becomes 'dust' (dust is a kind of dirt).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'fellow' becomes 'man' (fellow can mean a man).
'dust'+'man'='DUSTMAN'
(Other definitions for dustman that I've seen before include "Bin emptier" , "Refuse collector" , "My old man's occupation" , "Refuse worker" , "Garbage collector" .)