About five hundred birds in large groups (6)
I believe the answer is:
crowds
'large groups' is the definition.
The definition and answer can be both groups as well as being plural nouns.
Maybe they are linked in a way I don't understand?
'about five hundred birds' is the wordplay.
'about' indicates putting letters inside.
'five hundred' becomes 'D' (Roman numeral).
'birds' becomes 'crows' (crow is a kind of bird).
'd' going within 'crows' is 'CROWDS'.
'in' is the link.
Can you help me to learn more?
(Other definitions for crowds that I've seen before include "Hosts" , "Swarms" , "huge turnout?" , "Presses" , "Large gatherings of people" .)