Heard sports group swarm (4)
I believe the answer is:
teem
'swarm' is the definition.
('teem' can be a synonym of 'swarm')
'heard sports group' is the wordplay.
'heard' shows a homophone (sound like).
'sports group' becomes 'team'.
'team' is a homophone of 'TEEM'.
(Other definitions for teem that I've seen before include "Pullulate" , "(Of rain) pour down" , "Seethe" , "Rain buckets" , "Be full" .)