Bird closer to startled flock (5)
I believe the answer is:
crowd
'flock' is the definition.
(I've seen this before)
'bird closer to startled' is the wordplay.
'bird' becomes 'crow' (crow is a kind of bird).
'closer to' says to take the final letters (the letter which ends or closes the word).
The last letter of 'startled' is 'd'.
'crow'+'d'='CROWD'
(Other definitions for crowd that I've seen before include "Spectators" , "See 1" , "Large number of people gathered together" , "assembly" , "Army" .)