Usual food for a stationer? (6,4)

I believe the answer is:

staple diet

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

'usual food' is the definition.
'staple diet' can be an answer for 'food' (I have seen 'Principal food' mean 'staple diet' so perhaps 'food' could also mean 'staple diet'). I am not certain of the 'usual' bit.

I can't explain the rest of the clue.

Can you help me to learn more?

(Other definitions for staple diet that I've seen before include "Principal food" , "The food one normally eats" .)

I've seen this clue in The Daily Express.

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.