Characters help on mountain height (8)
I believe the answer is:
alphabet
'characters' is the definition.
(I've seen this before)
'help on mountain height' is the wordplay.
'help' becomes 'abet' (I've seen this before).
'on' means one lot of letters go next to another.
'mountain' becomes 'alp' (the Alps are a mountain range).
'height' becomes 'h' (abbreviation in geometry).
'alp'+'h'='alph'
'abet' after 'alph' is 'ALPHABET'.
(Other definitions for alphabet that I've seen before include "A to Z inclusive" , "A-Z" , "Set of characters" , "Group of characters" , "Whole set of letters" .)