Beach flock (5)
I believe the answer is:
terns
'beach flock' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for terns that I've seen before include "Sea bird with long forked tail" , "Aquatic birds" , "Rents out these birds" , "Sea birds with long forked tails" , "Seabirds" .)