Projectile, originally lost, is in the sea (9)
I believe the answer is:
ballistic
'projectile' is the definition.
(I've seen this before)
'originally lost is in the sea' is the wordplay.
'originally' suggests taking the first letters.
'in' is an insertion indicator.
'the sea' becomes 'baltic' (Baltic is an example).
The first letter of 'lost' is 'l'.
'l'+'is'='lis'
'lis' placed inside 'baltic' is 'BALLISTIC'.
(Other definitions for ballistic that I've seen before include "Winsome deer" , "Like missiles" , "To do with the flight of projectiles - basic lilt (anag)" , "Relating to projectiles and their flight" , "Moved by gravity" .)