Displeased with department's representative sample (5-7)
I believe the answer is:
cross section
'representative sample' is the definition.
(I've seen this before)
'displeased with department's' is the wordplay.
'displeased' becomes 'cross'.
'with' means one lot of letters go next to another.
'department' becomes 'section' (section can mean a department or area).
'cross'+'section'='CROSS-SECTION'
(Other definitions for cross section that I've seen before include "Representative sample" , "Fair sample" , "random sample" , "Representative group" , "St Helena's "invention"?" .)