Dreads writhing snakes (6)

I believe the answer is:

adders

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

'dreads writhing' is the wordplay.
'writhing' indicates anagramming the letters.
'dreads' with letters rearranged gives 'ADDERS'.

(Other definitions for adders that I've seen before include "Arithmetic snakes?" , "at the zoo?" , "sum? We do" , "Snakes are totters" , "Dreads (anag) -- snakes" .)

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

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