Old king goes outside, up to the pair (6)
I believe the answer is:
couple
'the pair' is the definition.
('couple' can be a synonym of 'pair')
'old king goes outside up' is the wordplay.
'old king' becomes 'cole' (I've seen this in another clue).
'goes outside' is an insertion indicator.
'cole' enclosing 'up' is 'COUPLE'.
'to' acts as a link.
(Other definitions for couple that I've seen before include "The pair to join together" , "Married pair; two forces causing rotation" , "Bonnie and Clyde, say" , "Duo, brace" , "Twosome" .)