She oversaw agriculture before entering civil service (5)
I believe the answer is:
ceres
'she oversaw agriculture' is the definition.
(Roman goddess of agriculture)
'before entering civil service' is the wordplay.
'before' becomes 'ere' (usually literary).
'entering' indicates putting letters inside.
'civil service' becomes 'cs'.
'ere' put into 'cs' is 'CERES'.
(Other definitions for ceres that I've seen before include "Roman goddess; dwarf planet" , "Roman goddess of agriculture" , "The largest asteroid" , "chief of agriculture" , "Head of agriculture" .)