Nest egg initials (3)
I believe the answer is:
ira
'nest egg initials' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for ira that I've seen before include "Brother of George Gershwin" , "Illegal organisation, initially" , "Militant republicans" , "George Gershwin's older brother, the lyricist" , "George 13 down's brother" .)