First Lady with concern ultimately quits (4)
I believe the answer is:
even
'quits' is the definition.
(as in 'call it quits')
'first lady with concern ultimately' is the wordplay.
'first lady' becomes 'eve' (first woman in the Bible).
'with' says to put letters next to each other.
'ultimately' suggests the final letters.
The last letter of 'concern' is 'n'.
'eve'+'n'='EVEN'
(Other definitions for even that I've seen before include "Constant" , "Going so far as to" , "Not bumpy" , "Flat, level" , "Plumb" .)