Expert cancelled visiting dump (6)
I believe the answer is:
boffin
'expert' is the definition.
(boffin means an expert)
'cancelled visiting dump' is the wordplay.
'cancelled' becomes 'off' (similar in meaning).
'visiting' means one lot of letters goes inside another.
'dump' becomes 'bin' (both can mean to throw in the rubbish).
'off' placed into 'bin' is 'BOFFIN'.
(Other definitions for boffin that I've seen before include "Technical expert" , "Back-room boy" , "Brainbox" , "(Government) scientist" , "Scientist (in the back room?)" .)