Birds back with first one descending to ground (5)
I believe the answer is:
terns
'birds back' is the definition.
'terns' can be an answer for 'birds' (I have seen 'Aquatic birds ' mean 'terns' so perhaps 'birds' could also mean 'terns'). I'm not sure about the 'back' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for terns that I've seen before include "Sea birds with long forked tails" , "Sea bird with long forked tail" , "Rents out these birds" , "Seabirds" , "Aquatic birds" .)