I sound like a cat climbing over tin houses, breaking in (9)
I believe the answer is:
irruption
'i' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'sound like a cat climbing over tin houses breaking in' is the wordplay.
'sound like a cat' becomes 'purr' (I've seen this before).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'over tin houses' becomes 'tio' (I can't explain this - if you can you should give a lot more credence to this answer).
'breaking' is an insertion indicator.
'purr' backwards is 'rrup'.
'rrup'+'tio'='rruptio'
'rruptio' going into 'in' is 'IRRUPTION'.
Can you help me to learn more?
(Another definition for irruption that I've seen is " breakthrough".)