Nest egg option, briefly (3)
I believe the answer is:
ira
'nest egg option briefly' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ira that I've seen before include "nationalists" , "Illegal organisation, initially" , "George Gershwin's older brother, the lyricist" , "Militant Irish nationalist organisation" , "Songwriter George's elder brother" .)