Mountain top following fault (5)
I believe the answer is:
sinai
'mountain' is the definition.
(I've seen this before)
'top following fault' is the wordplay.
'top' becomes 'ai' (resembles A1, term for excellent).
'following' says to put letters next to each other.
'fault' becomes 'sin' (I've seen this in other clues).
'ai' put after 'sin' is 'SINAI'.
(Other definitions for sinai that I've seen before include "Peninsula, mostly desert, at north end of Red Sea" , "Mountainous peninsula in Egypt" , "Deserted area" , "Egyptian desert" , "Egyptian location" .)