Pass on porky dog (6)
I believe the answer is:
Here is my best explanation:
'dog' is the definition.
(collie is a kind of dog)
'pass on porky' is the wordplay.
'pass' becomes 'col' (col is a term for a mountain pass).
'on' is a charade indicator (letters next to each other).
'porky' becomes 'lie' (I've seen this before).