Good little boy, but apt to bite! (4)

I believe the answer is:

gnat

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

'apt to bite' is the definition.
I can't tell whether this definition defines the answer.

'good little boy' is the wordplay.
'good little' becomes 'g' (abbreviation for good).
'boy' becomes 'nat' ().
'g'+'nat'='GNAT'

'but' is the link.

Can you help me to learn more?

(Other definitions for gnat that I've seen before include "small irritant" , "Insect often hovering over stagnant water" , "Stinging insect" , "Tiny biting insect" , "Small biter" .)

I've seen this clue in the Evening Standard.

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