Deal with swarm returning (4)
I believe the answer is:
meet
'deal with' is the definition.
(thesaurus)
'swarm returning' is the wordplay.
'swarm' becomes 'teem' ('teem' can be a synonym of 'swarm').
'returning' says the letters should be written in reverse.
'teem' in reverse letter order is 'MEET'.
(Other definitions for meet that I've seen before include "Join" , "Countryside assembly now perhaps not legal" , "Sporting encounter" , "Attend" , "Assemble" .)