Swarm's come together, moving north (4)
I believe the answer is:
teem
'swarm's' is the definition.
(synonyms)
'come together moving north' is the wordplay.
'come together' becomes 'meet' (to meet is to gather or come together).
'moving north' shows that the letters should be reversed in order (in a down clue, letters go up or 'north').
'meet' reversed gives 'TEEM'.
(Other definitions for teem that I've seen before include "Be prolific" , "Rain buckets" , "Swarm around meet" , "Rain hard" , "Rain cats and dogs" .)