Blackball backward thug caught going too far (7)
I believe the answer is:
boycott
'blackball' is the definition.
(I've seen this before)
'backward thug caught going too far' is the wordplay.
'backward' is a reversal indicator.
'thug' becomes 'yob' (I've seen this before).
'caught' becomes 'c' (cricket abbreviation).
'going too far' becomes 'OTT' (short for over the top).
'yob' back-to-front is 'boy'.
'boy'+'c'+'ott'='BOYCOTT'
(Other definitions for boycott that I've seen before include "send to Coventry" , "Refuse to deal with - outlaw" , "Blackball" , "Refuse to deal with (someone) as a protest" , "Trade embargo" .)