House Democrat in heavy defeat setback (5)
I believe the answer is:
tudor
'house' is the definition.
(royal house)
'democrat in heavy defeat setback' is the wordplay.
'democrat' becomes 'd'.
'in' is an insertion indicator.
'heavy defeat' becomes 'rout' (I've seen this before).
'setback' is a reversal indicator.
'd' inserted within 'rout' is 'rodut'.
'rodut' reversed gives 'TUDOR'.
(Other definitions for tudor that I've seen before include "Pertaining to the period 1485-1603" , "Royal family, fourteen eighty five to sixteen hundred and three" , "Style of architecture in 15th c. England" , "eg Henry VIII" , "Name of the English royal family of Elizabeth 1st" .)