Top grade hotel in mountain area (5)
I believe the answer is:
alpha
'top grade' is the definition.
(I've seen this before)
'hotel in mountain area' is the wordplay.
'hotel' becomes 'h' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' is an insertion indicator.
'mountain' becomes 'alp' (the Alps are a mountain range).
'area' becomes 'a' (maths abbreviation).
'alp'+'a'='alpa'
'h' placed within 'alpa' is 'ALPHA'.
(Other definitions for alpha that I've seen before include "very good rating" , "starter" , "piece of Greek writing" , "Brightest star" , "- - and Omega" .)