Mark prosecuted about being uninterrupted (9)
I believe the answer is:
sustained
'uninterrupted' is the definition.
(thesaurus)
'mark prosecuted about' is the wordplay.
'mark' becomes 'stain' (synonyms).
'prosecuted' becomes 'sued' (I've seen this before).
'about' is an insertion indicator.
'stain' placed within 'sued' is 'SUSTAINED'.
'being' acts as a link.
(Other definitions for sustained that I've seen before include "held up" , "Kept (something) going" , "bore" , "Steady" , "Kept going, supported" .)