Took legal action about slur being prolonged (9)
I believe the answer is:
sustained
'being prolonged' is the definition.
(I've seen this before)
'took legal action about slur' is the wordplay.
'took legal action' becomes 'sued' (I've seen this in another clue).
'about' is an insertion indicator.
'slur' becomes 'stain' (something damaging someone's reputation).
'sued' placed around 'stain' is 'SUSTAINED'.
(Other definitions for sustained that I've seen before include "Long drawn-out" , "bore" , "prolonged" , "Kept (something) going" , "Kept going, supported" .)