Small group not heard (4)
I believe the answer is:
knot
'small group' is the definition.
(I've seen this before)
'not heard' is the wordplay.
'heard' shows a homophone (sound like).
'not' sounds like 'KNOT'.
(Other definitions for knot that I've seen before include "Tight cluster (of people)" , "To secure < > bird" , "Entwine" , "Small sandpiper - speed at sea - hitch" , "Tether" .)