Serpent always wants more, it seems (5)

I believe the answer is:

adder

I'm a little stuck... Click here to teach me more about this clue!

'serpent' is the definition.
(I know that adder is a type of viper)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for adder that I've seen before include "Poisonous Brit. snake" , "Venemous snake" , "a poisonous type" , "1 of 21 [SNAKES]" , "Type of snake" .)

I've seen this clue in The Irish Independent.

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