George Gershwin's brother (3)
I believe the answer is:
ira
'george gershwin's brother' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for ira that I've seen before include "Illegal organisation, initially" , "Gershwin, say" , "Militant Irish nationalist organisation" , "nationalists" , "Songwriter George's elder brother" .)