Dirty ancient moat in ruins (11)
I believe the answer is:
contaminate
'dirty' is the definition.
(contaminating is a kind of dirtying)
'ancient moat in ruins' is the wordplay.
'in ruins' indicates an anagram.
'ancient'+'moat'='ancientmoat'
'ancientmoat' with letters rearranged gives 'CONTAMINATE'.
(Other definitions for contaminate that I've seen before include "Add impurities to" , "Taint" , "Add impurity to (something)" , "Make (something) impure" , "Pollute" .)