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' means one lot of letters go next to another.
'a bad' is an anagram indicator.
'that' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'act' is an anagram of 'tac'.
'tac'+'i'='taci'
't' put 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" .)