Flocks of birds heading for distant parts (6)
I believe the answer is:
crowds
'flocks' is the definition.
'birds heading for distant parts' is the wordplay.
'birds' becomes 'crows' (crow is a kind of bird).
'heading for' says to take the initial letters.
'parts' indicates putting letters inside (in the sense of cuts or separates).
The first letter of 'distant' is 'd'.
'crows' placed around 'd' is 'CROWDS'.
'of' is the link.
(Other definitions for crowds that I've seen before include "Hosts" , "Hordes" , "Swarms" , "Presses" , "huge turnout?" .)