A Greek mountain has no topsoil (5)
I believe the answer is:
alpha
'a' is the definition.
(the Greek letter A)
'greek mountain has no topsoil' is the wordplay.
I cannot quite see how this works, but
'mountain' could be 'alp' (the Alps are a mountain range) and 'alp' is found in the answer.
The remaining letters 'ha' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
(Other definitions for alpha that I've seen before include "Top star" , "Leading Greek character" , "leader in Athens, originally" , "Early testing stage for new software or hardware" , "high mark" .)