Confident when sued about source of revenue (7)
I believe the answer is:
assured
'confident' is the definition.
(similar in meaning)
'when sued about source of revenue' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'about' indicates putting letters inside.
'source of' says to take the initial letters (the start - or source - of the word).
The first letter of 'revenue' is 'r'.
'sued' enclosing 'r' is 'sured'.
'as'+'sured'='ASSURED'
(Other definitions for assured that I've seen before include "Informed with certainty and confidence" , "without doubt" , "Gave confidence to" , "Confident and self-possessed" , "Exhibiting confidence" .)