3 down
Internal
enemy
in
appropriate
position
here?
(5,9)
Internal enemy in appropriate position here? (5,9)
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The definition is at the beginning of the clue.
How the clue works
The straight definition is Internal enemy, which a fifth columnist certainly is. The playful twist comes from the whole phrase pointing at the grid itself — this entry (3D) actually sits in the fifth column, so it's literally a fifth columnist in its 'appropriate position here'.