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' shows that the letters should be reversed in order.
'in' indicates the answer is hidden within the clue.
'ari' can be found hidden inside 'paris'.
'ari' reversed gives '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" .)