Traitor that is captured becomes furious (5)
I believe the answer is:
irate
'furious' is the definition.
(I know that furious can be written as irate)
'traitor that is captured' is the wordplay.
'traitor' becomes 'rat' (rat can mean a treacherous person).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'captured' means one lot of letters goes inside another.
'rat' going inside 'ie' is 'IRATE'.
'becomes' acts as a link.
(Other definitions for irate that I've seen before include "Angry, annoyed" , "Annoyed, nettled" , "I tear away, being annoyed" , "become irritated" , "Full of 22" .)