Rain sees gathering retreating (4)
I believe the answer is:
teem
'rain' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for teem that I've seen before include "Rain hard" , "Pullulate" , "Seethe" , "Be swarming (with)" , "Rain buckets" .)