Most precipitous (8)
I believe the answer is:
steepest
'most precipitous' is the definition.
('steep' can be similar in meaning to 'precipitous')
This is all the clue.
(Other definitions for steepest that I've seen before include "Most expensive" , "Rising most sharply" , "most severe" , "With the sharpest incline" , "extremely dear" .)