Overindulge in jam? (5)
I believe the answer is:
stuff
I believe this clue is a double definition.
'overindulge' is the first definition.
(I know that overindulge can be written as stuff)
'jam?' is the second definition.
(I know that stuff is a more specific form of the action back up)
'in' acts as a link.
(Other definitions for stuff that I've seen before include "Material - things" , "Jam" , "facts" , "set with unspecified ingredients" , "Substance; material" .)