Soldier possibly supporting old court sector (6)
I believe the answer is:
octant
'sector' is the definition.
(one of eight sectors in something)
'soldier possibly supporting old court' is the wordplay.
'soldier possibly' becomes 'ant' (as in army ants).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'court' becomes 'ct'.
'o'+'ct'='oct'
'ant' put after 'oct' is 'OCTANT'.
(Other definitions for octant that I've seen before include "Device to measure angles to a celestial body" , "Eighth part of circle" , "45 degrees" , "segment of disc" .)