Meet form of swarm (4)
I believe the answer is:
teem
'swarm' is the definition.
(synonyms)
'meet form' is the wordplay.
'form' indicates anagramming the letters (a different form of the same letters).
'meet' with letters rearranged gives 'TEEM'.
'of' is the link.
(Other definitions for teem that I've seen before include "Move in large numbers" , "Rain buckets" , "Flow copiously" , "Swarm around meet" , "Rain hard" .)