Banker confused about line supported by unknown carmaker (4,4)

'banker confused' is the definition.
I know nothing about this answer so I can't judge whether this works.

'about line supported by unknown carmaker' is the wordplay.
I cannot really see how this works, but
'about' could be 'a' (abbreviation in Chambers) and 'a' is found within the answer.
'line' could be 'l' (used when specifying particular lines from a poem) and 'l' is found in the answer.
'unknown' could be 'z' (algebra) and 'z' is present in the answer.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?