The thousand referred to (4)
I believe the answer is:
them
'referred to' is the definition.
I don't know anything about this answer so I cannot judge whether this works.
'the thousand' is the wordplay.
'thousand' becomes 'm' (Roman numeral for a thousand).
'the'+'m'='THEM'
Can you help me to learn more?
(Other definitions for them that I've seen before include "Certainly not us, the other lot" , "Others" , "Perhaps, the Opposition" , "people objectively" , "(To/from) those people" .)