Foul places knight visits heaving dull sigh (9)

'foul places' is the definition.
(foul-smelling places)

'knight visits heaving dull sigh' is the wordplay.
'knight' becomes 'N' (chess abbreviation).
'visits' indicates putting letters inside (visit can mean to go into).
'heaving' is an anagram indicator (letters thrown or heaved around).
'dull'+'sigh'='dullsigh'
'dullsigh' with letters rearranged gives 'dughills'.
'n' placed into 'dughills' is 'DUNGHILLS'.