Topless French girls supporting Planet Earth in French port (10)
I believe the answer is:
marseilles
'french port' is the definition.
'topless french girls supporting planet earth' is the wordplay.
'topless' means to remove the first letter.
'french girls' becomes 'filles' ('girls' in French).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'planet' becomes 'mars' (Mars is an example).
'earth' becomes 'e'.
'filles' with its initial letter taken away is 'illes'.
'mars'+'e'='marse'
'illes' after 'marse' is 'MARSEILLES'.
'in' acts as a link.
(Other definitions for marseilles that I've seen before include "Southern French port" , "in France" , "Mediterranean port" , "port becomes visible" , "place in Provence" .)