Crossword Genius

In a city (not London) is nothing totally unimportant (8)

Ross

I believe the answer is:

derisory

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

'unimportant' is the definition.
Both the definition and answer are adjectives. Maybe they are linked in a way I don't understand?

'in a city not london is nothing totally' is the wordplay.
I cannot quite see how this works, but
'is' is within the answer.
'nothing' could be 'o' (looks like zero - 0) and 'o' is found within the answer.
The remaining letters 'derry' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for derisory that I've seen before include "Deserving of ridicule or scorn" , "Ridiculously inadequate" , "worthy of scorn?" , "Showing scorn or ridicule" , "pathetically small" .)

I've seen this clue in The Guardian.
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