Popeye's girl is going round with chaps – that's a bad sign (4,4)

'a bad sign' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'popeye's girl is going round with chaps' is the wordplay.
'popeye's girl' becomes 'Olive' (Olive Oyl, girlfriend of Popeye).
'is going round' says the letters should be written backwards.
'with' says to put letters next to each other.
'chaps' becomes 'men' (chap means a man).
'olive' in reverse letter order is 'evilo'.
'evilo'+'men'='EVIL OMEN'

'that's' is the link.

Can you help me to learn more?