Gather up rain, intensely (4)
I believe the answer is:
teem
'rain intensely' is the definition.
'teem' can be an answer for 'rain' (thesaurus). I am not certain of the 'intensely' bit.
'gather up' is the wordplay.
'gather' becomes 'meet' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'meet' back-to-front is 'TEEM'.
Can you help me to learn more?
(Other definitions for teem that I've seen before include "Pullulate" , "Flow copiously" , "Rain hard" , "(Of rain) fall heavily" , "(Of rain) pour down" .)