Additional bit of text rarely seen (5)
I believe the answer is:
extra
'additional' is the definition.
(extra means additional or surplus)
'bit of text rarely seen' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'EXTRA' is hidden in the letters of 'text rarely seen'.
(Other definitions for extra that I've seen before include "Left over" , "Spare" , "There's more" , "An additional thing" , "more than required" .)