Portrays ancient Britons alongside 500 English (7)
I believe the answer is:
depicts
'portrays' is the definition.
('depict' can be a synonym of 'portray')
'ancient britons alongside 500 english' is the wordplay.
'ancient britons' becomes 'picts' (I've seen this in another clue).
'alongside' says to put letters next to each other.
'500' becomes 'D' (Roman numeral for 500).
'english' becomes 'E' (abbreviation).
'd'+'e'='de'
'picts' put after 'de' is 'DEPICTS'.
(Other definitions for depicts that I've seen before include "outlines" , "Paints or draws something" , "Describes or shows as picture" .)