Swarmed (6)
I believe the answer is:
teemed
'swarmed' is the definition.
('teem' can be a synonym of 'swarm')
This is the entire clue.
(Other definitions for teemed that I've seen before include "Swarmed or poured" , "(Of rain) poured down" , "Was full of, swarmed" , "Swarmed - abounded" , "Swarmed, was very plentiful" .)