Moving against him, the writer succeeded in competition honouring Poseidon (8,5)

I believe the answer is:

isthmian games

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'moving against him the writer succeeded in competition honouring poseidon' is the definition.
The definition suggests a singular noun which matches the answer.

'moving against him the writer succeeded' is the wordplay.
'moving' is an anagram indicator (letters are moved around).
'the writer' becomes 'me' (the person who wrote the clues).
'succeeded' becomes 's' (genealogical abbreviation).
'against'+'him'='againsthim'
'againsthim' is an anagram of 'isthmianga'.
'isthmianga'+'me'+'s'='ISTHMIAN GAMES'

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I've seen this clue in The Times.

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