The villains did wrong in base fashion! (7)
I believe the answer is:
baddies
'the villains' is the definition.
I know nothing about this answer so I can't judge whether this works.
'did wrong in base fashion' is the wordplay.
'wrong' indicates anagramming the letters.
'in' means one lot of letters goes inside another.
'fashion' is an anagram indicator.
'did' with letters rearranged gives 'ddi'.
'base' with letters rearranged gives 'baes'.
'ddi' going within 'baes' is 'BADDIES'.
Can you help me to learn more?
(Another definition for baddies that I've seen is " Informal or childish villains".)