In a mess, having nothing for starter for lunch, so stepped out (7)

'stepped out' is the definition.
The definition and answer can be both to do with motion as well as being past participle verbs.
Perhaps you can see a link between them that I can't see?

'in a mess having nothing for starter for lunch' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is found within the answer.
'nothing' could be 'o' (looks like zero - 0) and 'o' is found in the answer.
This may be the basis of the clue (or it may be nonsense).

'so' acts as a link.

Can you help me to learn more?