Crest, you say, showing a dog (4)
I believe the answer is:
peke
'a dog' is the definition.
(short for Pekingese)
'crest you say' is the wordplay.
'crest' becomes 'peak' (synonyms).
'you say' indicates a 'sounds like' (homophone) clue.
'peak' is a homophone of 'PEKE'.
'showing' is the link.
(Other definitions for peke that I've seen before include "In short, it's a small short-legged dog" , "Small snub-nosed dog" , "Sort of lapdog" , "Breed of dog (shortened)" , "Small breed of dog" .)