First lady to be seated, in a sense (5)
I believe the answer is:
astor
'first lady' is the definition.
Both the answer and definition are singular nouns.
Perhaps you can see an association between them that I can't see?
'to be seated in a sense' is the wordplay.
'be seated in' is an insertion indicator.
'sense' becomes 'sr' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'sr'='asr'
'to' put into 'asr' is 'ASTOR'.
Can you help me to learn more?
(Other definitions for astor that I've seen before include "Female politician" , "First female MP" , "First woman to sit in the House of Commons, d. 1964" , "female parliamentarian" , "See 2" .)