A large, extremely perilous mountain range (4)
I believe the answer is:
alps
'mountain range' is the definition.
(Alps is an example)
'a large extremely perilous' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'perilous' with its middle removed is 'ps'.
'a'+'l'+'ps'='ALPS'
(Other definitions for alps that I've seen before include "Slap (anag.)" , "Slap about in mountains" , "high areas" , "Mountain system in south-central Europe" , "Swiss mountains" .)