Thus again is angry (4)
I believe the answer is:
sore
'angry' is the definition.
(sore can mean angry or resentful)
'thus again' is the wordplay.
'thus' becomes 'so' (synonyms).
'again' becomes 're' (prefix meaning 'again').
'so'+'re'='SORE'
'is' acts as a link.
(Other definitions for sore that I've seen before include "Hurt" , "Boil" , "Bruised" , "Feeling pain or informally angry" , "Spot of aggravation" .)