The last pair of you can take five hundred to the southeast to external facilities (8)
I believe the answer is:
outhouse
'the last pair of you can take five hundred to the southeast' is the definition.
I can't judge whether this definition defines the answer.
'external facilities' is the wordplay.
'external' becomes 'out' (similar in meaning).
'facilities' becomes 'house' (I can't justify this - if you can you should believe this answer much more).
'out'+'house'='OUTHOUSE'
'to' is the link.
Can you help me to learn more?
(Other definitions for outhouse that I've seen before include "External building" , "Building in the grounds of a house" , "Construction" , "Building near home" , "Smaller building separate from main one" .)