Two letters providing identification up above chart (7)
I believe the answer is:
digraph
'two letters' is the definition.
(term for two letters representing one sound)
'identification up above chart' is the wordplay.
'identification' becomes 'id'.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'above' is a charade indicator (letters next to each other).
'chart' becomes 'graph' (I've seen this before).
'id' written backwards gives 'di'.
'di'+'graph'='DIGRAPH'
'providing' is the link.
(Other definitions for digraph that I've seen before include "Such as ph with f sound" , "EU, perhaps?" .)