Poem SAS composed in city (6)
I believe the answer is:
odessa
'city' is the definition.
(Odessa is an example)
'poem sas composed in' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'composed' indicates anagramming the letters.
'in' is an insertion indicator.
'sas' with letters rearranged gives 'ssa'.
'ode' going within 'ssa' is 'ODESSA'.
(Other definitions for odessa that I've seen before include "Port city of Ukraine" , "Battleship Potemkin mutiny port" , "Seaport" , "Ukrainian naval base" , "Ukrainian Black Sea port" .)