Certain lap dog, informally (3)
I believe the answer is:
pom
'certain lap dog informally' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for pom that I've seen before include "Pomeranian dog" , "Brit abroad" , "Australian term for Englishman" , "Little dog" , "Limey in Oz" .)