Reportedly, one's smacked as a House supporter (7)
I believe the answer is:
yorkist
'supporter' is the definition.
(I have seen 'royal supporter' mean 'yorkist' so perhaps 'supporter' could also mean 'yorkist')
'reportedly one's smacked as a house' is the wordplay.
I cannot really understand how this works, but
'one's' could be 'is' (Roman numeral) and 'is' is found in the answer.
'house' could be 'york' (I've seen this before) and 'york' is present in the answer.
A single letter 't' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for yorkist that I've seen before include "White rose wearer" , "Wars of the Roses partisan" , "As royal supporter" , "On side of white rose" , "Lancastrian's enemy" .)