Monolith's reputation not changed in home counties (8,5)
I believe the answer is:
standing stone
'monolith's' is the definition.
'reputation not changed in home counties' is the wordplay.
'reputation' becomes 'standing' (I've seen this before**).
'changed' indicates an anagram.
'in' indicates putting letters inside.
'home counties' becomes 'se'.
'not' anagrammed gives 'ton'.
'ton' placed inside 'se' is 'stone'.
'standing'+'stone'='STANDING STONE'
(Other definitions for standing stone that I've seen before include "Ancient monument" , "Menhir" , "Large rock placed upright" .)