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Subsection
of
secret
summons
turned
around
rally
(6)
Subsection of secret summons turned around rally (6)
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The definition is at the end of the clue.
How the clue works
Right, the definition's rally, as in gather everyone together. For the wordplay, dig into "secRET SUMmons" - there's a chunk of letters buried in there, and reversing them (that's the "turned around" bit) spells out the answer.