Supposed breakdown occurs in Police Department (8)
I believe the answer is:
presumed
'supposed' is the definition.
(I've seen this before)
'breakdown occurs in police department' is the wordplay.
'breakdown' becomes 'resume' (a resume is a breakdown of someone's career).
'occurs in' is an insertion indicator.
'police department' becomes 'pd' (as in NYPD, LAPD etc).
'resume' going inside 'pd' is 'PRESUMED'.
(Other definitions for presumed that I've seen before include "Supposed" , "Taken for granted" , "expected" , "thought" , "Was impertinent" .)