Swarm to gather up (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' backwards is 'TEEM'.
'to' is the link.
(Other definitions for teem that I've seen before include "Be full" , "Rain buckets" , "Rain cats and dogs" , "Be prolific" , "(Of rain) fall heavily" .)