Neckline and reptile (6)
I believe the answer is:
turtle
'neckline and reptile' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for turtle that I've seen before include "that flies in 2nd" , "Large marine reptile (with a fashionable neck!)" , "Marine reptile with shell" , "Neckline and reptile" , "eg Razorback" .)