'Island' composed by Gershwin? (7)
I believe the answer is:
madeira
'island' is the definition.
(I know that Madeira is a island)
'composed by gershwin?' is the wordplay.
'composed' becomes 'made'.
'by' says to put letters next to each other.
'gershwin?' becomes 'ira' (lyricist Ira Gershwin, brother of George).
'made'+'ira'='MADEIRA'
(Other definitions for madeira that I've seen before include "Fortified Portuguese wine" , "cake producer" , "Rich, fortified white wine" , "Atlantic island, famed for its sweet wine" , "Sponge-cake" .)