Tiny part of moat ruined (4)
I believe the answer is:
atom
'tiny part' is the definition.
'atom' can be an answer for 'part' (I have seen 'Part of molecule ' mean 'atom' so perhaps 'part' could also mean 'atom'). I am not sure about the 'tiny' bit.
'moat ruined' is the wordplay.
'ruined' indicates anagramming the letters.
'moat' is an anagram of 'ATOM'.
'of' is the link.
Can you help me to learn more?
(Other definitions for atom that I've seen before include "Neutron and proton" , "Small amount of matter" , "Tiny particle, great bomb" , "An explosive particle as in bomb" , "A really tiny piece of something" .)