I think the specific term for this clue is 'faulty'. The fact that 'at sea' is doing double duty in a figurative sense for both wordplay and definition simply makes it worse.
A cryptic clue consists of a definition of the solution and a separate, alternative way of reaching that solution (normally a cryptic wordplay, but on occasion a second, distinct definition). The exception is an all-in-one (&lit) clue where the entire clue stands as the definition, such as Colin Dexter's classic "Item gran arranged family slides in" for MAGIC LANTERN (CLAN 'sliding into' an arrangement of ITEM GRAN).
This clue should read something like 'Soldier insect at sea, drifting' or, if you think 'at sea' can mean 'directionless' (which I don't), 'Soldier insect adrift at sea'.