Large gatherings of birds - about 500 (6)
I believe the answer is:
'large gatherings' is the definition.
'crowds' can be an answer for 'gatherings' (crowd is a kind of gathering). I am 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' is the 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" .)