It follows a bad act, that's understood (5)
I believe the answer is:
tacit
'understood' is the definition.
(I've seen this before)
'it follows a bad act that's' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'follows' says to put letters next to each other.
'a bad' indicates an anagram.
'that' becomes 'i' (I can't explain this - if you can you should believe this answer much more).
'act' is an anagram of 'tac'.
'tac'+'i'='taci'
't' after 'taci' is 'TACIT'.
Can you help me to learn more?
(Other definitions for tacit that I've seen before include "Implied without being stated" , "Unspoken but implied" , "Unvoiced" , "Silently understood" , "Implied without being said" .)