Freshwater diving bird (5)
I believe the answer is:
grebe
'freshwater diving bird' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for grebe that I've seen before include "Diver" , "Swimmer" , "Duck-sized water-bird" , "Diving bird which may be crested" , "Aquatic bird that builds floating nests" .)