Dominant mountain height near Austria's capital (5)
I believe the answer is:
alpha
'dominant' is the definition.
(I've seen this in another clue)
'mountain height near austria's capital' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'height' becomes 'h' (abbreviation in geometry).
'near' is a charade indicator (letters next to each other).
'capital' says to take the initial letters.
The initial letter of 'austria' is 'a'.
'alp'+'h'+'a'='ALPHA'
(Other definitions for alpha that I've seen before include "First, best quality" , "First in order of importance (for a Greek?)" , "very good rating" , "Top star" , "best score" .)