Bird, first of dozens in flock (5)
I believe the answer is:
crowd
'flock' is the definition.
(I've seen this before)
'bird first of dozens' is the wordplay.
'bird' becomes 'crow' (crow is a kind of bird).
'first of dozens' becomes 'd' (cst letter of 'dozens').
'crow'+'d'='CROWD'
'in' acts as a link.
(Other definitions for crowd that I've seen before include "Multitude" , "Large number of people in one place" , "Large group of people" , "Masses" , "Spectators" .)