After a bad act I become silent (8)
I believe the answer is:
taciturn
'silent' is the definition.
(taciturn means not saying much)
'after a bad act i become' is the wordplay.
'after' is a charade indicator (letters next to each other).
'a bad' indicates an anagram.
'become' becomes 'turn' (synonyms).
'act' with letters rearranged gives 'tac'.
'tac' next to 'i' is 'taci'.
'taci'+'turn'='TACITURN'
(Other definitions for taciturn that I've seen before include "Disinclined towards conversation" , "Tight-lipped" , "Disinclined to speak" , "Far from talkative" , "Without saying much" .)