Girl saves boy living by the sea (8)
I believe the answer is:
maritime
'by the sea' is the definition.
(I've seen this before)
'girl saves boy living' is the wordplay.
'girl' becomes 'Marie' (girl's name).
'saves' is an insertion indicator.
'boy living' becomes 'tim' (I can't justify this - if you can you should believe this answer much more).
'marie' enclosing 'tim' is 'MARITIME'.
(Other definitions for maritime that I've seen before include "Relating to the sea" , "Naval" , "Coastal" , "Of seafaring or shipping" , "Concerned with the main" .)