Weighty responsibility saving deposit up (7)
I believe the answer is:
onerous
'weighty' is the definition.
(I've seen this before)
'responsibility saving deposit up' is the wordplay.
'responsibility' becomes 'onus' (I've seen this before).
'saving' is an insertion indicator.
'deposit' becomes 'ore' (I have seen 'Deposit of mine' mean 'ore' so perhaps 'deposit' could also mean 'ore').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'ore' back-to-front is 'ero'.
'onus' enclosing 'ero' is 'ONEROUS'.
(Other definitions for onerous that I've seen before include "Oppressive, difficult to bear" , "Burdensome, weighty" , "Involving much work" , "Burdensome, taxing" , "Trying" .)