Stage includes the end that's precipitous (5)
I believe the answer is:
steep
'that's precipitous' is the definition.
(similar in meaning)
'stage includes the end' is the wordplay.
'stage' becomes 'step' (as in a step or stage of a process).
'includes' is an insertion indicator.
'end' says to take the final letters.
The final letter of 'the' is 'e'.
'step' enclosing 'e' is 'STEEP'.
(Other definitions for steep that I've seen before include "Precipitous - outrageous" , "Sharply inclined" , "Sheer, precipitous" , "Unreasonably dear - soak" , "Rapidly rising" .)