Swarms encounter rising sun initially (5)
I believe the answer is:
teems
'swarms' is the definition.
('teem' can be a synonym of 'swarm')
'encounter rising sun initially' is the wordplay.
'encounter' becomes 'meet' (synonyms).
'rising' says the letters should be written in reverse (letters written upwards in a down clue).
'sun initially' becomes 's' (abbreviation. I am not sure about the 'initially' bit.).
'meet' back-to-front is 'teem'.
'teem'+'s'='TEEMS'
Can you help me to learn more?
(Other definitions for teems that I've seen before include "Abounds, swarms" , "Pours down" , "Rains heavily" , "Buckets down" .)