One engaged in breaking down or treating breakdowns (7)
I believe the answer is:
analyst
'treating breakdowns' is the definition.
I can't judge whether this defines the answer.
'one engaged in breaking down' is the wordplay.
'one' becomes 'an' (an apple is one apple).
'engaged in' becomes 'at' (to be 'at' a task is to be engaged in doing it).
'breaking' means one lot of letters goes inside another (inserted letters break into others).
'down' becomes 'lys' (I can't justify this - if you can you should believe this answer much more).
'at' going around 'lys' is 'alyst'.
'an'+'alyst'='ANALYST'
'or' is the link.
Can you help me to learn more?
(Other definitions for analyst that I've seen before include "Person who interprets system structures" , "Examiner - psychiatrist" , "Practical chemist" , "One identifying (chemical) composition" , "One who determines chemical composition" .)