Does it augment the zoo stock? (5)

I believe the answer is:

adder

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

'does it augment the zoo stock?' is the definition.
I can't judge whether this defines the answer.

'augment the zoo stock?' is the wordplay.
'augment' becomes 'add' (I've seen this before).
'the zoo stock?' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'add'+'er'='ADDER'

Can you help me to learn more?

(Other definitions for adder that I've seen before include "Poisonous Brit. snake" , "Venomous sort" , "Totter" , "poisoner" , "One contributing more" .)

I've seen this clue in the Evening Standard.

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