Reptiles with bony shell and flipper-like limbs (7)
I believe the answer is:
turtles
'reptiles with bony shell and flipper-like limbs' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for turtles that I've seen before include "Let rust settle on sea-creatures" , "The reptiles are let rust" , "Swimming reptiles" , "Marine reptiles" , "Slow swimmers" .)