25 across
Following
one
action
of
union,
embracing
it
(9)
Following one action of union, embracing it (9)
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The definition is at the beginning of the clue.
How the clue works
Following is the definition, meaning imitating. Take I for "one" and MATING for "action of union", then have that word embrace (contain) IT for "it" — slotting IT inside MATING gives M-IT-ATING, which after the leading I spells IMITATING.