Bird finally joined flock (5)
I believe the answer is:
crowd
'flock' is the definition.
(I've seen this before)
'bird finally joined' is the wordplay.
'bird' becomes 'crow' (crow is a kind of bird).
'finally' indicates one should take the final letters.
The final letter of 'joined' is 'd'.
'crow'+'d'='CROWD'
(Other definitions for crowd that I've seen before include "Masses" , "Large group of people" , "Army" , "Horde" , "Social set" .)