Reckon one’s travelling by car? (8)
I believe the answer is:
estimate
'reckon' is the definition.
(estimating is a kind of reckoning)
'one's travelling by car?' is the wordplay.
I cannot really understand how this works, but
'one's' could be 'im' ('one is' is a formal way of saying 'I am') and 'im' is located in the answer.
'by' could be 'at' and 'at' is found within the answer.
'car?' could be 't' and 't' is found in the answer.
This explanation may well be incorrect...
(Other definitions for estimate that I've seen before include "Believe" , "Guess the cost" , "damage" , "Statement giving the likely cost of a job" , "Form opinion of - tea times (anag)" .)