Tiny part damaged moat (4)
I believe the answer is:
atom
'tiny part' is the definition.
(I've seen this in another clue)
'damaged moat' is the wordplay.
'damaged' is an anagram indicator.
'moat' anagrammed gives 'ATOM'.
(Other definitions for atom that I've seen before include "Not much substance" , "Neutron and proton" , "Small particle of matter" , "Smallest particle in an element able to take part in a chemical reaction" , "little amount" .)