Riddle is about the first lady (5)
I believe the answer is:
sieve
'riddle' is the definition.
(thesaurus)
'is about the first lady' is the wordplay.
'about' shows that the letters should be reversed in order.
'the first' means one lot of letters go next to another.
'lady' becomes 'eve' (I've seen this before**).
'is' written backwards gives 'si'.
'si'+'eve'='SIEVE'
(Other definitions for sieve that I've seen before include "Craft for Jumblies" , "Utensil for straining or sifting food" , "Fine strainer" , "Screen, sift, strain" , "Separate" .)