Black cats and dogs, one thinks (5)
I believe the answer is:
brain
'black cats and dogs one thinks' is the definition.
I can't judge whether this defines the answer.
'black cats and dogs one' is the wordplay.
'black cats' becomes 'bra' (I can't explain this - if you can you should give a lot more credence to this answer).
'and' becomes 'n' (common abbreviation for 'and').
'dogs' means one lot of letters go next to another (dogs can mean follows or chases).
'one' becomes 'I' (Roman numeral).
'n' put after 'i' is 'in'.
'bra'+'in'='BRAIN'
Can you help me to learn more?
(Other definitions for brain that I've seen before include "It's in the skull" , "Clever person" , "Intellectual ability" , "Organ of the intellect" , "Intelligent person" .)