Stop a barrier crushing brother (8)
I believe the answer is:
abrogate
'stop' is the definition.
(I've seen this before)
'a barrier crushing brother' is the wordplay.
'barrier' becomes 'gate' (gate is a kind of barrier).
'crushing' means one lot of letters goes inside another (inserted letters are crushed by others).
'brother' becomes 'bro' (short for brother).
'a'+'gate'='agate'
'agate' placed around 'bro' is 'ABROGATE'.
(Other definitions for abrogate that I've seen before include "Annul or revoke" , "Get rid of" , "Abolish formally" , "Formally cancel (an agreement)" , "Rescind" .)