Squad heard to swarm (4)
I believe the answer is:
teem
'to swarm' is the definition.
('teem' can be a synonym of 'swarm')
'squad heard' is the wordplay.
'squad' becomes 'team' (synonyms).
'heard' shows a homophone (sound like).
'team' sounds like 'TEEM'.
(Other definitions for teem that I've seen before include "Be full" , "Rain cats and dogs" , "Abound, swarm with" , "Swarm around meet" , "Rain buckets" .)