Reign was marred by bad language (8)
I believe the answer is:
swearing
'bad language' is the definition.
(I've seen this before)
'reign was marred' is the wordplay.
'marred' indicates anagramming the letters.
'reign'+'was'='reignwas'
'reignwas' anagrammed gives 'SWEARING'.
'by' acts as a link.
(Other definitions for swearing that I've seen before include "Foul language" , "Affirming or cursing" , "blue emissions?" , "Vowing" , "Expressing annoyance" .)