Nitric oxide canister unavailable (3,2)

I believe the answer is:

not in

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

'unavailable' is the definition.
I know nothing about this answer so I cannot tell whether this works.

'nitric oxide canister' is the wordplay.
'nitric oxide' becomes 'NO' (NO is the chemical formula for nitric oxide).
'canister' becomes 'tin' (synonyms).
'no'+'tin'='NOT IN'

Can you help me to learn more?

(Other definitions for not in that I've seen before include "Out or away, house empty" , "ie Out" .)

I've seen this clue in the Sydney Morning Herald.

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.