Early seventeenth century woman astride a sturdy horse (8)
I believe the answer is:
jacobean
'early seventeenth century' is the definition.
(relating to the reign of James I, 16031625)
'woman astride a sturdy horse' is the wordplay.
'woman' becomes 'Jean' (woman's name).
'astride' indicates putting letters inside.
'sturdy horse' becomes 'cob' (a cob is a powerfully built horse).
'a'+'cob'='acob'
'jean' enclosing 'acob' is 'JACOBEAN'.
(Other definitions for jacobean that I've seen before include "of 17th century design" , "Of period when James I reigned" , "of Scottish king" , "Relating to the 1603-1625 period" , "From the time of James I" .)