Shut up and take notice — this group's bigger than ABBA (6)
I believe the answer is:
pentad
'this group's bigger than abba' is the definition.
(group of five things)
'shut up and take notice' is the wordplay.
'shut up' becomes 'pent' (synonyms).
'and take' is a charade indicator (letters next to each other).
'notice' becomes 'ad' (both can mean an advertisement).
'pent'+'ad'='PENTAD'
(Other definitions for pentad that I've seen before include "quintet" , "small group" , "Half a decade" , "Enid Blyton's was famous" , "Set of five things" .)