Deduced, when one got angry about (8)
I believe the answer is:
reasoned
'deduced' is the definition.
(I've seen this before)
'when one got angry about' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'got' means one lot of letters go next to another.
'angry' becomes 'red' (someone may go red when angry).
'about' is an insertion indicator.
'as'+'one'='asone'
'asone' put within 'red' is 'REASONED'.
(Other definitions for reasoned that I've seen before include "Performed brain operations" , "Debated in calm and logical way" , "Based on logic" , "sound" , "Worked out through logic and sense" .)