Said to be in pairs - before, that is (3)
I believe the answer is:
ere
'before that is' is the definition.
'ere' can be an answer for 'before' (poetic term for 'before'). I'm not sure about the remainder of the definition.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for ere that I've seen before include "Before, poetically" , "Before (lit.)" , "Old word for before" , "Archaic word for before - a palindrome" , "Old prior" .)