Pass fib on to dog (6)
I believe the answer is:
collie
'to dog' is the definition.
(collie is a kind of dog)
'pass fib on' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'fib' becomes 'lie' (a fib is a lie).
'on' means one lot of letters go next to another.
'col'+'lie'='COLLIE'
(Other definitions for collie that I've seen before include "Working animal" , "Silky-coated sheepdog" , "Sheepdog with long pointed muzzle" , "Popular sheepdog like Lassie" , "Scottish sheep dog" .)