Return to encounter swarm (4)
I believe the answer is:
teem
'swarm' is the definition.
(I know that swarm can be written as teem)
'return to encounter' is the wordplay.
'return' shows that the letters should be reversed in order.
'to encounter' becomes 'meet' (synonyms).
'meet' back-to-front is 'TEEM'.
(Other definitions for teem that I've seen before include "Move in large numbers" , "Swarm around meet" , "numerous" , "(Of rain) fall heavily" , "(Of rain) pour down" .)