Peak stuff, something that sounds a warning (10)
I believe the answer is:
matterhorn
'peak' is the definition.
(Matterhorn is a mountain)
'stuff something that sounds a warning' is the wordplay.
'stuff' becomes 'matter' (I've seen this in another clue).
'something that sounds a warning' becomes 'horn' (as in a car horn).
'matter'+'horn'='MATTERHORN'
(Other definitions for matterhorn that I've seen before include "Mountain in the Alps" , "A scale that may be difficult" , "one of a range" , "peak" , "Alpine mountain" .)