Bird in green ready to ascend (4)
I believe the answer is:
erne
'bird' is the definition.
(I know that erne is a type of sea eagle)
'in green ready to ascend' is the wordplay.
'in' indicates the answer is hidden within the clue.
'to ascend' is a reversal indicator (I've seen 'ascending' mean this).
'enre' can be found hidden inside 'green ready'.
'enre' written backwards gives 'ERNE'.
(Other definitions for erne that I've seen before include "Sea-eagle in an Irish lake" , "Seabird" , "White-tailed eagle" , "Sea bird: lough in Ulster" , "A high-flier" .)