Setting a trap by perjuring one fellow, backing the ex-President (5,2,6)

I believe the answer is:

lying in ambush

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'perjuring one fellow backing the ex-president' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.

'setting a trap' is the wordplay.
'setting' becomes 'lyingin' (I can't justify this - if you can you should believe this answer much more).
'a trap' becomes 'ambush' ('ambush' can be a synonym of 'trap').
'lyingin'+'ambush'='LYING IN AMBUSH'

'by' is the link.

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I've seen this clue in the Sydney Morning Herald.

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