Leave conclusions on top-level damage (5,4)
I believe the answer is:
split ends
'top-level damage' is the definition.
Although both the answer and 'damage' are nouns, they are not in the same form of the noun.
'leave conclusions on' is the wordplay.
'leave' becomes 'split' (I've seen this before).
'conclusions' becomes 'ends' (end is a kind of conclusion).
'on' means one lot of letters go next to another.
'split'+'ends'='SPLIT ENDS'
Can you help me to learn more?
(Other definitions for split ends that I've seen before include "Slip tends to create unsightly hair condition" , "damaged hair" , "signs of poor hair" , "Hair problem" , "hairy problem" .)