Tunnel (6)
I believe the answer is:
burrow
'tunnel' is the definition.
(I know that tunnel can be written as burrow)
This is all the clue.
(Other definitions for burrow that I've seen before include "Hole dug by a small animal" , "Rabbit's home" , "Animal's hole in the ground" , "A rabbit's tunnel" , "Dig into the ground like a rabbit" .)