Mountain, say, separated from sierra (4)
I believe the answer is:
peak
'mountain' is the definition.
(the peak of a mountain)
'say separated from sierra' is the wordplay.
'say' becomes 'speak' (synonyms).
'separated from' suggests deleting specific letters.
'sierra' becomes 'S' (phonetic alphabet: alpha, bravo, charlie etc.).
'speak' with 's' taken out is 'PEAK'.
(Other definitions for peak that I've seen before include "Apex" , "Part of cap; mountain" , "Pointed mountain" , "Sharp pointed summit" , "maximum pitch" .)