Plodding (6)
I believe the answer is:
leaden
'plodding' is the definition.
(I know that plodding can be written as leaden)
This is all the clue.
(Other definitions for leaden that I've seen before include "Leaned over in slow, heavy way" , "Depressing - heavy" , "Heavy - overcast" , "Slow-moving, being like heavy metal" , "Heavy - plodding" .)