Potential success is in doughnuts or fried onions with salt (6,4)

'potential success' is the definition.
(a rising star is likely to have great success)

'is in doughnuts or fried onions with salt' is the wordplay.
I cannot really understand how this works, but
'is' is within the answer.
'in' is found in the answer.
'salt' could be 'tar' (both can informally mean a sailor) and 'tar' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?