Superior unions collectively back strike (8)
I believe the answer is:
uppercut
'strike' is the definition.
(strike with your fist)
'superior unions collectively back' is the wordplay.
'superior' becomes 'upper' (eg an upper house in a parliament).
'unions collectively' becomes 'TUC' (Trades Union Congress).
'back' shows that the letters should be reversed in order.
'tuc' back-to-front is 'cut'.
'upper'+'cut'='UPPERCUT'
(Other definitions for uppercut that I've seen before include "Swinging blow in boxing" , "Punch to opponent's chin from below" , "Blow from fist to chin" , "Boxing blow delivered to chin from below" , "Swinging blow directed at an opponent's chin" .)