Portrayed ancient Briton in action (8)
I believe the answer is:
depicted
'portrayed' is the definition.
(synonyms)
'ancient briton in action' is the wordplay.
'ancient briton' becomes 'pict' (ancient people in Scotland during Roman times).
'in' is an insertion indicator.
'action' becomes 'deed' (deed is a kind of action).
'pict' inserted within 'deed' is 'DEPICTED'.
(Other definitions for depicted that I've seen before include "Described or portrayed in words" , "Shown" .)