Assumed European articles were upright (10)
I believe the answer is:
understood
'assumed' is the definition.
('understood' can be a synonym of 'assumed')
'european articles were upright' is the wordplay.
'european articles' becomes 'under' (as in part-of-speech. un+der).
'were upright' becomes 'stood' ('stand' can be a synonym of 'be upright').
'under'+'stood'='UNDERSTOOD'
(Other definitions for understood that I've seen before include "Accepted, implied" , "Got" , "Taken as 23 [READ]" , "Comprehended" .)