Walk slowly and heavily (6)
I believe the answer is:
trudge
'walk slowly and heavily' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for trudge that I've seen before include "Plod doggedly" , "Walk heavily, as if through mud" , "Walk wearily" , "Walk with slow, heavy steps" , "Footslog" .)