Air composed by George Gershwin's brother (3)
I believe the answer is:
ira
'george gershwin's brother' is the definition.
(I've seen this before)
'air composed' is the wordplay.
'composed' indicates anagramming the letters.
'air' with letters rearranged gives 'IRA'.
'by' acts as a link.
(Other definitions for ira that I've seen before include "George Gershwin's older brother, the lyricist" , "terrorists" , "Illegal organisation, initially" , "nationalists" , "Songwriter George's elder brother" .)