Eco warrior originally arrested under nuclear dump (6,3)
I believe the answer is:
number two
'dump' is the definition.
(I've seen this before)
'eco warrior originally arrested under nuclear' is the wordplay.
'eco' becomes 'Umberto' (Italian author Umberto Eco).
'originally' says to take the initial letters.
'arrested' is an insertion indicator (inserted letters are held or arrested).
'under' is a charade indicator (letters next to each other) (in a down clue, some letters go below others).
'nuclear' becomes 'N' (abbreviation).
The initial letter of 'warrior' is 'w'.
'umberto' placed around 'w' is 'umbertwo'.
'umbertwo' after 'n' is 'NUMBER TWO'.
(Other definitions for number two that I've seen before include "Assistant" , "Second in command" , "deputy" , "vice" , "Second-in-command" .)