Graceful fliers
I believe the answer is:
'graceful fliers' 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 "Aquatic birds" , "Sea birds with long forked tails" , "Seabirds" , "Rents out these birds" , "Sea bird with long forked tail" .)