They have an odd number of wheels (6)

'wheels' is the definition.
The answer and definition can be both man-made objects as well as being plural nouns.
Perhaps you can see a link between them that I can't see?

'they have an odd number' is the wordplay.
I cannot quite see how this works, but
'an' could be 'i' (Roman numeral for one) and 'i' is found within the answer.
'number' could be 'k' (K is an example) and 'k' is present in the answer.
The remaining letters 'tres' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).

'of' acts as a link.

Can you help me to learn more?