Groundwork by graduate with half-sister (5)
I believe the answer is:
basis
'groundwork' is the definition.
('basis' can be a synonym of 'groundwork')
'graduate with half-sister' is the wordplay.
'graduate' becomes 'ba'.
'with' means one lot of letters go next to another.
'half' indicates taking half.
'sister' cut in half is 'sis'.
'ba'+'sis'='BASIS'
'by' acts as a link.
(Other definitions for basis that I've seen before include "Ground, foundation" , "Principal element" , "Form of support" , "grounds" , "Foundation, fundament" .)