Marine reptile (3,5)
'marine reptile' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for turtle that I've seen before include "Reptile with bony shell" , "Sea swimmer" , "marine animal" , "that flies in 2nd" , "Shelled marine creature" .)
(Other definitions for sea snake that I've seen before include "Venomous little thing" , "Venomous type" , "Venomous marine creature" .)