Badly treat five hundred — and four … (6)
I believe the answer is:
tetrad
'four' is the definition.
(synonyms)
'badly treat five hundred' is the wordplay.
'badly' is an anagram indicator.
'five hundred' becomes 'd' (synonyms).
'treat' is an anagram of 'tetra'.
'tetra'+'d'='TETRAD'
'and' acts as a link.
(Other definitions for tetrad that I've seen before include "quartet" , "number of Gospel writers" , "Group or set of four" .)