Beach souvenir (3)
'beach souvenir' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for tan that I've seen before include "Light brown colour" , "Yellowish-brown colour" , "Thrash" , "Make black and blue" , "Shade of brown" .)
(Other definitions for shell that I've seen before include "Outer covering of 1 across, say" , "Hard casing; a racing boat" , "(Turtle's) carapace" , "pepper?" , "Protective outer case" .)