Into underground chamber river cut a shape (5)
I believe the answer is:
carve
'cut a shape' is the definition.
(carve a shape into something)
'into underground chamber river' is the wordplay.
'into' indicates putting letters inside.
'underground chamber' becomes 'cave' (I've seen this before).
'river' becomes 'r' (abbreviation).
'cave' going around 'r' is 'CARVE'.
(Other definitions for carve that I've seen before include "Cut (cooked meat)" , "Cut into pieces" , "produced by desire, with a shifting to the left" , "Chisel, sculpt" , "Slice (joint)" .)