Clever clogs offloading with a barrow, once (5)
I believe the answer is:
knoll
'barrow once' is the definition.
(barrow was a historical term for a mound or knoll)
I don't understand the remainder of the clue.
(Other definitions for knoll that I've seen before include "Small rounded hill" , "Round hillock" , "Hummock" , "Small round hill" , "minor eminence" .)