Quiet don marred the Atlantic (4)
I believe the answer is:
pond
'atlantic' is the definition.
(as in 'across the pond')
'quiet don marred' is the wordplay.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'marred' indicates an anagram.
'don' anagrammed gives 'ond'.
'p'+'ond'='POND'
'the' is the link.
(Other definitions for pond that I've seen before include "Small lake" , "Possible site for weed" , "Small body of water" , "Small area of still water" , "Pool of still water" .)