Anything turning with variable note, like an old police siren (3,4)

'like an old police siren' is the definition.
(police sirens traditionally used a two tone siren)

'anything turning with variable note' is the wordplay.
'anything' becomes 'owt' (owt means anything in Northern English dialects).
'turning' says the letters should be written backwards.
'with' means one lot of letters go next to another.
'variable' is an anagram indicator (a varied form of the letters).
'owt' reversed gives 'two'.
'note' with letters rearranged gives 'tone'.
'two'+'tone'='TWO-TONE'