Trial over agreement involving Republican (6)
I believe the answer is:
ordeal
'trial' is the definition.
(thesaurus)
'over agreement involving republican' is the wordplay.
'over' is a charade indicator (letters next to each other).
'agreement' becomes 'deal' (deal is a kind of agreement).
'involving republican' becomes 'or' (I am not sure about this - if you are sure you should believe this answer much more).
'deal' put after 'or' is 'ORDEAL'.
Can you help me to learn more?
(Other definitions for ordeal that I've seen before include "Loader (anag.)" , "Trial; nightmare" , "Long painful experience" , "Difficult and painful experience" , "Unpleasant and prolonged experience" .)