11 across
In
December
on
retreat,
large
corporation
is
in
fancy
dress
(8)
In December on retreat, large corporation is in fancy dress (8)
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The definition is at the end of the clue.
How the clue works
Look at in fancy dress — that's your definition. Now for the wordplay: DEC (short for December) is reversed (on retreat) to give CED, and tucked inside that you place OS (large) followed by TUM (corporation — slang for stomach), giving you C-OSTUM-ED.