Supplementary text rambles a bit (5)
I believe the answer is:
extra
'supplementary' is the definition.
(thesaurus)
'text rambles a bit' is the wordplay.
'a bit' says the answer is hidden in the clue (the answer is formed by a bit of the clue).
'EXTRA' is hidden in the letters of 'text rambles'.
(Other definitions for extra that I've seen before include "more than is necessary" , "Left over" , "Additional or supplementary" , "appearing in crowd scene" , "someone acting in a crowd" .)