Bay tree's outer surface (4)
I believe the answer is:
bark
This clue is a double definition.
'bay' is the first definition.
(I've seen this before)
'tree's outer surface' is the second definition.
(Other definitions for bark that I've seen before include "Sound of dog - protector of trunks" , "Sailing-ship" , "Skin of tree" , "A dog's shout" , "Sound from animal < having part of tree" .)