That many were treated badly was important (8)
I believe the answer is:
mattered
'was important' is the definition.
(to matter is to be important)
'that many were treated badly' is the wordplay.
I cannot quite understand how this works, but
'many' could be 'm' (Roman numeral for a thousand) and 'm' is present in the answer.
an anagram of 'treated' is 'attered' which is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mattered that I've seen before include "Counted, had weight" , "Counted, had significance" , "Was of importance" , "Was significant" .)