Fashionable department at hard comprehensive (2,5)
I believe the answer is:
in depth
'comprehensive' is the definition.
(I've seen this before)
'fashionable department at hard' is the wordplay.
'fashionable' becomes 'in' (in can mean fashionable or trendy).
'department' becomes 'dept'.
'at' means one lot of letters go next to another.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'in'+'dept'+'h'='IN-DEPTH'
(Other definitions for in depth that I've seen before include "full" , "Detailed sort of investigation" , "Not 16 [SHALLOW], anyway" , "Thoroughly and comprehensively" , "comprehensive" .)