Little dog, briefly (3)
I believe the answer is:
pom
'little dog briefly' is the definition.
(Pom is short for Pomeranian; 'briefly' asks for an abbreviation)
This is all the clue.
(Other definitions for pom that I've seen before include "Small dog (colloq.)" , "Shortened name for a breed of dog" , "Australian slang for British migrant" , "Limey in Oz" , "Little dog" .)