Essential matter overlooked by group as key moment in a court? (3,5)
I believe the answer is:
set point
'key moment in a court?' is the definition.
(as in a tennis court)
'essential matter overlooked by group' is the wordplay.
'essential matter' becomes 'point' (as in phrases like 'that's not the point').
'overlooked by' means one lot of letters go next to another (in a down clue, some letters go above others).
'group' becomes 'set' (both can mean a collection).
'point' after 'set' is 'SET POINT'.
'as' acts as a link.
(Other definitions for set point that I've seen before include "important opportunity in court" , "Close to victory here" , "ready to engrave one name into trophy?" , "Final score needed to win a tennis division" , "one crucial to serve" .)