Meet up and be prolific (4)
I believe the answer is:
teem
'be prolific' is the definition.
(I've seen this before)
'meet up' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'meet' back-to-front is 'TEEM'.
'and' acts as a link.
(Other definitions for teem that I've seen before include "Pullulate" , "Flow copiously" , "stream" , "Bucket down" , "numerous" .)