Be quiet about group's sluggishness (5)
I believe the answer is:
sloth
'sluggishness' is the definition.
(sloth is laziness)
'be quiet about group's' is the wordplay.
'be quiet' becomes 'sh' (both are said to tell someone to stop speaking).
'about' is an insertion indicator.
'group' becomes 'lot'.
'sh' enclosing 'lot' is 'SLOTH'.
(Other definitions for sloth that I've seen before include "One's often suspended from tree" , "Sluggish creature" , "Indolence" , "Laziness; slow mammal" , "One hangs a lot" .)