Amazing bird concealing tail in nest (9)
I believe the answer is:
startling
'amazing' is the definition.
(I've seen this in another clue)
'bird concealing tail in nest' is the wordplay.
'bird' becomes 'starling' (starling is a kind of bird).
'concealing' means one lot of letters goes inside another.
'tail in' says to take the final letters.
The final letter of 'nest' is 't'.
'starling' placed around 't' is 'STARTLING'.
(Other definitions for startling that I've seen before include "Surprising, shocking" , "Surprising or alarming" , "Extraordinary" , "Causing shock" , "Causing surprise" .)