Subject to regular current variation (5)

I believe the answer is:

tidal

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'subject' is the definition.
Although both the answer and definition are adjectives, I cannot see how they can define each other.

I don't understand the rest of the clue.

Can you help me to learn more?

(Other definitions for tidal that I've seen before include "__ wave; tsunami" , "What a bore may be" , "Type of wave" , "Subject to moon's pull" , "(Waters) subject to moon/sun" .)

I've seen this clue in The Globe and Mail.

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