Substance from Istanbul kiosk (4)

I believe the answer is:

bulk

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

'substance' is the definition.
Although both the answer and definition are singular nouns, I can't understand how one could define the other.

'from istanbul kiosk' is the wordplay.
'from' indicates a hidden word.
'BULK' is hidden in the letters of 'istanbul kiosk'.

Can you help me to learn more?

(Other definitions for bulk that I've seen before include "Amplitude" , "Mass - large quantity" , "Great size; main part" , "Majority" , "Large mass; roughage" .)

I've seen this clue in The Sun.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.