It follows a bad act, that's understood (5)
I believe the answer is:
tacit
'understood' is the definition.
(I've seen this in another clue)
'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 anagramming the letters.
'that's' becomes 'i' (I am not sure about this - if you are sure you should believe this answer much more).
'act' anagrammed gives '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" .)