Black lagoon swirling in city (7)
I believe the answer is:
bologna
'city' is the definition.
(Bologna is an example)
'black lagoon swirling in' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'swirling' is an anagram indicator (letters get swirled around).
'in' means one lot of letters goes inside another.
'lagoon' anagrammed gives 'ologna'.
'b' placed inside 'ologna' is 'BOLOGNA'.
(Other definitions for bologna that I've seen before include "North Italian university city" , "Capital of Emilia-Romagna" , "Italian city - smoked sausage" , "Italian province" .)