One augmenting the reptile population (5)

I believe the answer is:

adder

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'one augmenting the reptile population' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.

This is the entire clue.

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(Other definitions for adder that I've seen before include "Venomous sort" , "Small poisonous snake, viper" , "1 of 21 [SNAKES]" , "a poisonous type" , "exhibit in the reptile house" .)

I've seen this clue in the Evening Standard.

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