Gather up in swarm (4)
I believe the answer is:
teem
'swarm' is the definition.
('teem' can be a synonym of 'swarm')
'gather up' is the wordplay.
'gather' becomes 'meet' ('meet' can be a synonym of 'gather').
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'meet' back-to-front is 'TEEM'.
'in' is the link.
(Other definitions for teem that I've seen before include "Swarm around meet" , "Be swarming (with)" , "Rain buckets" , "Bucket down" , "(Of rain) fall heavily" .)