Swarm to one side, say (4)
I believe the answer is:
teem
'swarm' is the definition.
(I know that swarm can be written as teem)
'one side say' is the wordplay.
'one side' becomes 'team' (a side is a sporting team).
'say' shows a homophone (sound like).
'team' sounds like 'TEEM'.
'to' acts as a link.
(Other definitions for teem that I've seen before include "Rain cats and dogs" , "(Of rain) pour down" , "Bear" , "Abound, swarm with" , "numerous" .)