I sound like a cat climbing over tin houses, breaking in (9)
I believe the answer is:
irruption
'breaking in' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'i sound like a cat climbing over tin houses' is the wordplay.
'sound like a cat' becomes 'purr' (I've seen this before).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'over tin houses' becomes 'tion' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'purr' written backwards gives 'rrup'.
'i'+'rrup'+'tion'='IRRUPTION'
Can you help me to learn more?
(Another definition for irruption that I've seen is " breakthrough".)