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 an anagram.
'air' anagrammed gives 'IRA'.
'by' is the 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" .)