Snakes less happy, beginning to slide to the bottom (6)

I believe the answer is:

adders

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'snakes' is the definition.
(adder is a kind of snake)

I don't understand the remainder of the clue.

(Other definitions for adders that I've seen before include "at the zoo?" , "Do they deal with tots" , "Mathematical snakes" , "sum? We do" , "Re Dad's snakes" .)

I've seen this clue in The Guardian.

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