Stand in for Republican first lady penning tale (7)
I believe the answer is:
relieve
'stand in for' is the definition.
(thesaurus)
'republican first lady penning tale' is the wordplay.
'republican' becomes 'r' (abbreviation).
'first lady' becomes 'eve' (first woman in the Bible).
'penning' means one lot of letters goes inside another.
'tale' becomes 'lie' (tale can mean a lie or invention).
'eve' placed around 'lie' is 'elieve'.
'r'+'elieve'='RELIEVE'
(Other definitions for relieve that I've seen before include "Alleviate (eg, pain)" , "Ease pain or burden" , "Comfort" , "Assuage, ease" , "Lighten a burden or raise a seige" .)