Monsieur in rebuilt Ostend terminal (7)
I believe the answer is:
endmost
'terminal' is the definition.
(synonyms)
'monsieur in rebuilt ostend' is the wordplay.
'monsieur' becomes 'm' (French abbreviation - like Mr for Mister).
'in' means one lot of letters goes inside another.
'rebuilt' is an anagram indicator.
'ostend' with letters rearranged gives 'endost'.
'm' placed inside 'endost' is 'ENDMOST'.
(Other definitions for endmost that I've seen before include "Most distant" , "Last in the line" , "final" , "Furthest away" , "extreme situation" .)