Opener once in abstract game
I believe the answer is:
drawbridge
'opener' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's a link between them I don't understand?
'once in abstract game' is the wordplay.
'once in abstract' becomes 'draw' (I can't justify this - if you can you should believe this answer much more).
'game' becomes 'bridge' (bridge is a kind of game).
'draw'+'bridge'='DRAWBRIDGE'
Can you help me to learn more?
(Other definitions for drawbridge that I've seen before include "adjustable spanner" , "One may protect castle" , "Keep opening" , "Barbican may defend this" , "that's raised by defenders" .)