Donkey wide in tunnel (6)
I believe the answer is:
burrow
'tunnel' is the definition.
(I know that tunnel can be written as burrow)
'donkey wide' is the wordplay.
'donkey' becomes 'burro' (term for a small donkey).
'wide' becomes 'w' (abbreviation e.g. in cricket).
'burro'+'w'='BURROW'
'in' acts as a link.
(Other definitions for burrow that I've seen before include "Dig underground" , "Warren" , "Tunnel; animal shelter" , "Rabbit's home" , "Animal's excavation" .)