Showcase who's missing a great pair? (4)

I believe the answer is:

aces

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'showcase who's missing a' is the definition.
I can't tell whether this defines the answer.

'great pair?' is the wordplay.
'great' becomes 'ace' (I've seen this before).
'pair?' becomes 's' (I can't justify this - if you can you should believe this answer much more).
'ace'+'s'='ACES'

Can you help me to learn more?

(Other definitions for aces that I've seen before include "Champions" , "never come back" , "holes in one" , "that are non-returnable" , "excellent service" .)

I've seen this clue in the Irish Times.

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