Justification for conflict with Republican worker (7)
I believe the answer is:
warrant
'justification' is the definition.
'conflict with republican worker' is the wordplay.
'conflict' becomes 'war' (war is a kind of conflict).
'with' means one lot of letters go next to another.
'republican' becomes 'r' (abbreviation).
'worker' becomes 'ant' (worker is a type of ant).
'war'+'r'+'ant'='WARRANT'
'for' acts as a link.
(Other definitions for warrant that I've seen before include "Guaranty - authorisation to make an arrest" , "Permit for arrest, search of premises, etc" , "Call for" , "Document giving authority" , "Travel pass" .)