On return, encounter swarm (4)
I believe the answer is:
teem
'swarm' is the definition.
(synonyms)
'on return encounter' is the wordplay.
'on return' is a reversal indicator.
'encounter' becomes 'meet' (synonyms).
'meet' reversed gives 'TEEM'.
(Other definitions for teem that I've seen before include "Swarm around meet" , "Rain cats and dogs" , "Seethe" , "Be full" , "Be swarming (with)" .)