Port returned song with thanks (5)
I believe the answer is:
yalta
'port' is the definition.
(I've seen this before)
'returned song with thanks' is the wordplay.
'returned' shows that the letters should be reversed in order.
'song' becomes 'lay' (poetic term for a song).
'with' means one lot of letters go next to another.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'lay' reversed gives 'yal'.
'yal'+'ta'='YALTA'
(Other definitions for yalta that I've seen before include "Nineteen forty five conference site" , "Crimean resort and seaport" , "Ukrainian port" , "WW2 1945 conference town" , "City on the Black Sea, where Roosevelt, Stalin and Churchill met in 1945" .)