After bad act I become silent (8)
I believe the answer is:
taciturn
'silent' is the definition.
(thesaurus)
'after bad act i become' is the wordplay.
'after' says to put letters next to each other.
'bad' indicates an anagram.
'become' becomes 'turn' ('turn' can be a synonym of 'become'**).
'act' anagrammed gives 'tac'.
'tac' put next to 'i' is 'taci'.
'taci'+'turn'='TACITURN'
(Other definitions for taciturn that I've seen before include "Reticent" , "Reserved, saying little" , "Untalkative" , "Habitually silent" , "Reserved and uncommunicative" .)