One on the subject of anger (3)
I believe the answer is:
ire
'anger' is the definition.
(ire is intense rage)
'one on the subject of' is the wordplay.
'one' becomes 'i' (Roman numeral).
'on the subject of' becomes 're' (re means regarding a certain topic).
'i'+'re'='IRE'
(Other definitions for ire that I've seen before include "Anger, rage" , "passion" , "'Anger, wrath (3)'" , "Losing head" , "Hot temper" .)