Sad in France, English staying in seaside city (7)
I believe the answer is:
trieste
'seaside city' is the definition.
'sad in france english staying in' is the wordplay.
'sad in france' becomes 'triste' (I've seen this before).
'english' becomes 'e' (abbreviation).
'staying in' means one lot of letters goes inside another.
'triste' placed around 'e' is 'TRIESTE'.
(Other definitions for trieste that I've seen before include "Italian border city" , "Italian city, James Joyce's 13" , "NE Italy port" , "European city" , "Italy port, near Slovenia" .)