House here, in Marseilles, by the sea (6)
I believe the answer is:
medici
'house' is the definition.
(I know that Medici is a type of house)
'here in marseilles by the sea' is the wordplay.
'here in marseilles' becomes 'ici' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'the sea' becomes 'med' (short for Mediterranean).
'ici' put after 'med' is 'MEDICI'.
(Other definitions for medici that I've seen before include "Name of great Florentine family 15-17th centuries" , "Florentine ruling family" , "Ice mid the Italian dynasty" , "Florentine ruler" , "house in Florence" .)