Type of missile, large, is found in sea (9)
I believe the answer is:
'type of missile' is the definition.
'large is found in sea' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'found in' is an insertion indicator.
'sea' becomes 'baltic' (Baltic is an example).
'lis' going within 'baltic' is 'BALLISTIC'.
(Other definitions for ballistic that I've seen before include "Moved by gravity", "Projectile", "Relating to projectiles and their flight", "Like missiles", "To do with the flight of projectiles - basic lilt (anag)".)