A considerable number with a capacity to believe - slaughtered (9)
I believe the answer is:
massacred
'slaughtered' is the definition.
('massacre' can be a synonym of 'slaughter')
'a considerable number with a capacity' is the wordplay.
'a considerable number' becomes 'mass' (I've seen this in another clue).
'with' means one lot of letters go next to another.
'a capacity' becomes 'acred' (I can't justify this - if you can you should believe this answer much more).
'mass'+'acred'='MASSACRED'
'to believe' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for massacred that I've seen before include "butchered" , "Indiscriminately killed" , "treated appallingly" .)