The First Lady, 50, seen in New York regularly (6)
I believe the answer is:
evenly
'regularly' is the definition.
('evenly' can be a synonym of 'regularly')
'the first lady 50 seen in new york' is the wordplay.
'the first lady' becomes 'eve' (I've seen this before**).
'50' becomes 'l' (roman numeral).
'seen in' indicates putting letters inside.
'new york' becomes 'ny'.
'l' placed into 'ny' is 'nly'.
'eve'+'nly'='EVENLY'
(Other definitions for evenly that I've seen before include "In measured or level manner" , "In level way or manner" , "at regular intervals" , "In smooth or level manner" , "In equal proportion" .)