Heard mountain dog (4)
I believe the answer is:
peke
'dog' is the definition.
(short for Pekingese)
'heard mountain' is the wordplay.
'heard' indicates a 'sounds like' (homophone) clue.
'mountain' becomes 'peak' (the peak of a mountain).
'peak' is a homophone of 'PEKE'.
(Other definitions for peke that I've seen before include "In short, it's a small short-legged dog" , "Little dog" , "Small canine" , "Snub-nosed lap-dog (colloq.)" , "Breed of dog (shortened)" .)