Ancient city holding acidity award (6)
I believe the answer is:
trophy
'award' is the definition.
(I know that trophy is a type of award)
'ancient city holding acidity' is the wordplay.
'ancient city' becomes 'troy' (Troy was an ancient city).
'holding' is an insertion indicator.
'acidity' becomes 'ph'.
'troy' placed around 'ph' is 'TROPHY'.
(Other definitions for trophy that I've seen before include "Memento of success" , "Victor's prize" , "leading award" , "Winner's prize" , "Object symbolising success" .)