Absolutely backed up - depend on it! (6)
I believe the answer is:
purely
'absolutely' is the definition.
(thesaurus)
'backed up depend on it' is the wordplay.
'backed' says the letters should be written backwards.
'depend' becomes 'rely' (I've seen this before).
'on it' is a charade indicator (letters next to each other).
'up' back-to-front is 'pu'.
'pu'+'rely'='PURELY'
(Other definitions for purely that I've seen before include "Wholly, unconditionally" , "Entirely; chastely" , "Simply and solely" , "Simply; utterly" , "Wholly or merely" .)