Rejected eggs containing a flaw (5)
I believe the answer is:
stain
'flaw' is the definition.
(thesaurus)
'rejected eggs containing a' is the wordplay.
'rejected' is a reversal indicator.
'eggs' becomes 'nits' (nit is a kind of egg).
'containing' indicates putting letters inside.
'nits' enclosing 'a' is 'niats'.
'niats' in reverse letter order is 'STAIN'.
(Other definitions for stain that I've seen before include "Unsightly mark" , "Tarnish" , "Smear" , "Discoloured mark" , "Discolouration" .)