Meet up and fall heavily (4)
I believe the answer is:
teem
'fall heavily' is the definition.
(I've seen this before)
'meet up' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'meet' written backwards gives 'TEEM'.
'and' is the link.
(Other definitions for teem that I've seen before include "Be prolific" , "Flow copiously" , "Come down hard" , "Swarm around meet" , "Rain hard" .)