Just depend on this setter? On the contrary! (6)
I believe the answer is:
merely
'just' is the definition.
('merely' can be a synonym of 'just')
'depend on this setter? on the contrary' is the wordplay.
'depend' becomes 'rely' (I've seen this before).
'on' means one lot of letters go next to another.
'this setter? on the contrary' becomes 'me' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rely' put after 'me' is 'MERELY'.
Can you help me to learn more?
(Other definitions for merely that I've seen before include "Just one" , "Simply - only" , "'Only, and nothing more (6)'" .)