It describes movement, in the main (5)

I believe the answer is:

tidal

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'main' is the definition.
Although both the answer and definition are adjectives, I can't understand how they can define each other.

I don't understand the remainder of the clue.

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(Other definitions for tidal that I've seen before include "Concerning lunar-driven ocean movements" , "(Waters) subject to moon/sun" , "Relating to ocean surges" , "Ebbing and flooding" , "Of sea, rising and falling" .)

I've seen this clue in The Telegraph.

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