Get together in turn and be in large numbers (4)
I believe the answer is:
teem
'be in large numbers' is the definition.
(teem can mean to swarm with)
'get together in turn' is the wordplay.
'get together' becomes 'meet' (meeting is a kind of getting together).
'in turn' is a reversal indicator (letters turning around).
'meet' back-to-front is 'TEEM'.
'and' acts as a link.
(Other definitions for teem that I've seen before include "numerous" , "(Of rain) pour down" , "Swarm around meet" , "Abound - pour" , "Move in large numbers" .)