One used to think of black cats and dogs (5)
I believe the answer is:
brain
'one used to think' is the definition.
I can't tell whether this definition defines the answer.
'black cats and dogs' is the wordplay.
'black' becomes 'b' (as in pencil classifications).
'cats and dogs' becomes 'rain' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'b'+'rain'='BRAIN'
'of' is the link.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Thinking thing" , "Mind" , "Intellectual capacity" , "Organ of the intellect" , "and 15 ac. The loss of experts through emigration" .)