How does this work?

declare @test-2 table (Col1 varchar(50))

insert into @test-2

select '[abcdef] Client' UNION ALL

select '[gdhdnuidd] 1st Party SW' UNION ALL

select 'testbusiness'

select case when charindex(']', Col1) > 0 then substring(Col1, Charindex(']', Col1)+2, 50)

else Col1 end

from @test-2