Some tommyrot, arrant, holding up storyteller (8)
I believe the answer is:
narrator
'storyteller' is the definition.
(I know that storyteller can be written as narrator)
'some tommyrot arrant holding up' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'rotarran' can be found hidden inside 'tommyrot arrant holding'.
'rotarran' in reverse letter order is 'NARRATOR'.
(Other definitions for narrator that I've seen before include "The one who tells the story" , "Raconteur" , "Soryteller" , "Story teller" , "He tells the story - arrant or what?" .)