Large gatherings of birds - about 500 (6)
I believe the answer is:
crowds
'large gatherings' is the definition.
'crowds' can be an answer for 'gatherings' (crowd is a kind of gathering). I'm not certain of the 'large' bit.
'birds about 500' is the wordplay.
'birds' becomes 'crows' (crow is a kind of bird).
'about' is an insertion indicator.
's00' becomes 'D' (Roman numeral for s00).
'crows' placed around 'd' is 'CROWDS'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for crowds that I've seen before include "Large gatherings of people" , "Swarms" , "huge turnout?" , "Presses" , "Hosts" .)