Open up, as you might say, an old letter (6)
I believe the answer is:
uncial
'an old letter' is the definition.
(historical type of written script)
'open up as you might say' is the wordplay.
'open up' becomes 'unseal' (remove a seal).
'as you might say' indicates a 'sounds like' (homophone) clue.
'unseal' sounds like 'UNCIAL'.
(Other definitions for uncial that I've seen before include "Lu, I can make a type of script" , "Nail CU to a form of old Greek and Latin writing" .)