Party after triumph with ultimate in chess openings (7)
I believe the answer is:
windows
'openings' is the definition.
(I know this)
'party after triumph with ultimate in chess' is the wordplay.
'party' becomes 'do' (do can informally mean a party).
'after' says to put letters next to each other.
'triumph' becomes 'win' (a win is a victory or triumph).
'with' becomes 'w' (short for with).
'ultimate in' suggests the final letters.
The final letter of 'chess' is 's'.
'do' after 'win' is 'windo'.
'windo'+'w'+'s'='WINDOWS'
(Other definitions for windows that I've seen before include "openings" , "Microsoft operating system" , "Computer operating system with a graphical user interface" .)