Old hag damaged front of monument showing an ancient alphabet (5)
I believe the answer is:
ogham
'an ancient alphabet' is the definition.
(I've seen this before)
'old hag damaged front of monument' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'damaged' indicates an anagram.
'front of' indicates taking the first letters.
The first letter of 'monument' is 'm'.
'hag' is an anagram of 'gha'.
'o'+'gha'+'m'='OGHAM'
'showing' is the link.
(Other definitions for ogham that I've seen before include "Old Irish alphabet with letters of lines and notches" , "Old Irish alphabetical system with lines and notches in stone" , "set of characters" , "Old British characters" , "Old Irish writing system of lines and notches in stone" .)