Display old hard part I brought in (7)
I believe the answer is:
exhibit
'display' is the definition.
(I know that display can be written as exhibit)
'old hard part i brought in' is the wordplay.
'old' becomes 'ex' (prefix meaning former or onetime).
'hard' becomes 'h' (abbreviation used in pencil classifications).
'part' becomes 'bit' (both mean a piece of something).
'brought in' is an insertion indicator.
'h'+'bit'='hbit'
'hbit' placed around 'i' is 'hibit'.
'ex'+'hibit'='EXHIBIT'
(Other definitions for exhibit that I've seen before include "Manifest" , "Put on display" , "Put on show" , "Show - piece of evidence in court" , "Object on formal public display" .)