Nothing interrupts the saints - that's a propitious sign (4,4)

'a propitious sign' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'nothing interrupts the saints' is the wordplay.
'nothing' becomes 'O' (looks like zero - 0).
'interrupts' means one lot of letters goes inside another.
'the saints' becomes 'goodmen' (I can't justify this - if you can you should give a lot more credence to this answer).
'o' inserted inside 'goodmen' is 'GOOD OMEN'.

'that's' acts as a link.

Can you help me to learn more?