Old English letter seen in archive throughout (3)
I believe the answer is:
eth
'old english letter' is the definition.
(I've seen this before)
'in archive throughout' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ETH' can be found hidden inside 'archive throughout'.
'seen' acts as a link.
(Other definitions for eth that I've seen before include "dated letter" , "Little Ethel" , "18's fiance" , "girl" , "Old English letter" .)