Riddle of island backed by first lady (5)
I believe the answer is:
sieve
'riddle' is the definition.
(thesaurus)
'island backed by first lady' is the wordplay.
'island' becomes 'is'.
'backed' shows that the letters should be reversed in order.
'by' is a charade indicator (letters next to each other).
'first lady' becomes 'eve' (I've seen this before**).
'is' written backwards gives 'si'.
'si'+'eve'='SIEVE'
'of' acts as a link.
(Other definitions for sieve that I've seen before include "Colander" , "Straining device" , "Screen, sift, strain" , "Kitchen strainer" , "Device for separating lumps" .)