Unconventional subject in Prague once (8)
I believe the answer is:
bohemian
I believe this clue is a double definition.
'unconventional' is the first definition.
(similar in meaning)
'subject in prague once' is the second definition.
(the Kingdom of Bohemia included Prague)
(Other definitions for bohemian that I've seen before include "Nonconformist" , "Artistic, unconventional" , "Unconventional type" , "of old country" , "Unconventional - like King Wenceslas!" .)