Halves of beer to begin with one’s put round party (7)
I believe the answer is:
bisects
'halves' is the definition.
'beer to begin with one's put round party' is the wordplay.
'to begin with' says to take the initial letters.
'one's' becomes 'is' (Roman numeral).
'put round' means one lot of letters goes inside another.
'party' becomes 'sect' (I've seen this before).
The initial letter of 'beer' is 'b'.
'is' enclosing 'sect' is 'isects'.
'b'+'isects'='BISECTS'
'of' acts as a link.
(Other definitions for bisects that I've seen before include "Cuts in two" , "Cuts in half" , "Divides in equal halves" , "Divides into two parts" .)