Porthole with a gap is welcome (5)

I believe the answer is:

hullo

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

'is welcome' is the definition.
(I've seen this before)

I cannot understand how the rest of the clue works.

(Another definition for hullo that I've seen is " welcome".)

I've seen this clue in The Independent.

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.