First Lady's new uniform (4)
I believe the answer is:
even
'uniform' is the definition.
(thesaurus)
'first lady's new' is the wordplay.
'first' means one lot of letters go next to another.
'lady' becomes 'eve' (I've seen this before**).
'new' becomes 'n'.
'eve' put next to 'n' is 'EVEN'.
(Other definitions for even that I've seen before include "Still - flat" , "Smooth, uniform in quality" , "Level, calm" , "Still - divisible by two" , "Not bumpy" .)