Fail to appreciate foreign articles one's imported by car (13)
I believe the answer is:
underestimate
'fail to appreciate' is the definition.
(underestimate can mean to fail to appreciate)
'foreign articles one's imported by car' is the wordplay.
'foreign articles' becomes 'under'.
'one's' becomes 'I'm' ('one is' is a formal way of saying 'I am').
'imported by' means one lot of letters goes inside another.
'car' becomes 'estate' (type of car).
'im' inserted inside 'estate' is 'estimate'.
'under'+'estimate'='UNDERESTIMATE'
(Other definitions for underestimate that I've seen before include "Regard (someone) as less capable than is the case" , "Evaluation that is too low" , "inaccurate" , "Sell short" , "Low guess" .)