27 across
Red,
pace
ahead
of
blue
(7)
Red, pace ahead of blue (7)
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The definition is at the beginning of the clue.
How the clue works
The definition is Red — Trotsky being a famous communist, i.e. a "red". Then the wordplay: TROT (a pace) placed ahead of SKY (blue, as in the colour of a clear sky), and the two together give you TROTSKY.