Invalid Newcastle University students? (4)

I believe the answer is:

null

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

'invalid' is the definition.
(similar in meaning)

'newcastle university students?' is the wordplay.
'newcastle university' becomes 'nu' (this might be a standard abbreviation I've not previously seen).
'students?' becomes 'll' (an abbreviation for learner driver written twice).
'nu'+'ll'='NULL'

Can you help me to learn more?

(Other definitions for null that I've seen before include "Without force, validity" , "Immaterial and void" , "Amounting to nothing" , "Useless - nothing" , "Invalid - not binding" .)

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.