Brute and villain played on unknown weakness (13)
I believe the answer is:
vulnerability
'weakness' is the definition.
(vulnerability is a kind of weakness)
'brute and villain played on unknown' is the wordplay.
'and' says to put letters next to each other (I've seen this in other clues).
'played' indicates anagramming the letters.
'on' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'brute' after 'villain' is 'villainbrute'.
'villainbrute' anagrammed gives 'vulnerabilit'.
'vulnerabilit'+'y'='VULNERABILITY'
(Other definitions for vulnerability that I've seen before include "Openness to injury" , "Weakness in defences - tiny blue rival (anag)" .)