Atheneum regulars appear in great numbers (4)
I believe the answer is:
teem
'appear in great numbers' is the definition.
I can't judge whether this definition defines the answer.
'atheneum regulars' is the wordplay.
'regulars' indicates alternate letters (letters selected at regular intervals).
The alternate letters of 'atheneum' are 'TEEM'.
Can you help me to learn more?
(Other definitions for teem that I've seen before include "Rain buckets" , "(Of rain) fall heavily" , "Abound, swarm with" , "Rain cats and dogs" , "Be full" .)