Poor workers getting mistreated (con men set it up, bypassing union) (12)

'poor workers' is the definition.
(people who are bad or poor at their jobs)

'mistreated con men set it up bypassing union' is the wordplay.
'mistreated' indicates an anagram.
'bypassing' suggests deleting specific letters (to bypass is to avoid).
'union' becomes 'U' (common abbreviation as in 'Trades Union Congress').
'con'+'men'+'set'+'it'+'up'='conmensetitup'
'conmensetitup' with 'u' taken away is 'conmensetitp'.
'conmensetitp' with letters rearranged gives 'INCOMPETENTS'.

'getting' acts as a link.