...where drivers do their level best? (15)

'where drivers do' is the definition.
The definition suggests a singular noun but the answer is not.

'level best?' is the wordplay.
I cannot really understand how this works, but
'level' could be 'a' (A level is an example) and 'a' is found within the answer.
'best?' could be 'a' and 'a' is located in the answer.
This may be the basis of the clue (or it may be nonsense).

'their' is the link.

Can you help me to learn more?