Overindulge in the valley (5)
I believe the answer is:
gorge
I believe this is a double definition.
'overindulge' is the first definition.
(I know that overindulge can be written as gorge)
'the valley' is the second definition.
(gorge is a kind of valley)
'in' acts as a link.
(Other definitions for gorge that I've seen before include "Gobble" , "Overeat in the deep ravine" , "Devour" , "chine" , "In which a river may flow" .)