Variety of dahlia (6)
(Other definitions for pompom that I've seen before include "Anti-aircraft cannon - woolly ball" , "Decorative ball of tufted wool" , "Woolly decoration" , "fluffy decoration" , "Top decoration" .)
'variety of dahlia' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for pom-pom that I've seen before include "Woollen clothing ornament" , "Fluffy decorative ball" , "Woollen ball" , "Variety of dahlia" , "Decorative tuft on shoes etc." .)