Obsessive sibling takes year for breakdown (8)
I believe the answer is:
analysis
'breakdown' is the definition.
(I've seen this before)
'obsessive sibling takes year' is the wordplay.
'obsessive' becomes 'anal' (anal can mean obsessed with order).
'sibling' becomes 'sis' (short for sister).
'takes' means one lot of letters goes inside another.
'year' becomes 'y'.
'anal'+'sis'='analsis'
'analsis' placed around 'y' is 'ANALYSIS'.
'for' is the link.
(Other definitions for analysis that I've seen before include "See 15" , "Examination of structure (eg of psyche)" , "Investigation of the component parts which make up the whole" , "Breakdown (for study)" , "Criticism" .)