Gershwin back in Paris (3)
I believe the answer is:
ira
'gershwin' is the definition.
(lyricist Ira Gershwin, brother of George)
'back in paris' is the wordplay.
'back' says the letters should be written in reverse.
'in' indicates the answer is hidden within the clue.
'ari' is hidden in the letters of 'paris'.
'ari' back-to-front is 'IRA'.
(Other definitions for ira that I've seen before include "Illegal organisation, in short" , "Gershwin, say" , "Illegal organisation, initially" , "Songwriter George's elder brother" , "nationalists" .)