Prep for a marathon, with "up" (4)
I believe the answer is:
carb
'prep for a marathon with up' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for carb that I've seen before include "Substance such as sugar or starch in foods" , "Carburettor (abbrev.)" , "Fuel" .)