A mountain? Don't be ridiculous! (5)
I believe the answer is:
alpha
'a' is the definition.
(the Greek letter A)
'mountain? don't be ridiculous' is the wordplay.
'mountain?' becomes 'alp' (the Alps are a mountain range).
'don't be ridiculous' becomes 'ha' (I can't explain this - if you can you should give a lot more credence to this answer).
'alp'+'ha'='ALPHA'
Can you help me to learn more?
(Other definitions for alpha that I've seen before include "First in order of importance (for a Greek?)" , "Foremost" , "high mark" , "Brightest star" , "Brilliant star" .)