One I might bang in new, unopened prison (4)
I believe the answer is:
nail
'one i might bang in' is the definition.
(eg you might bang a nail into a wall)
'new unopened prison' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'unopened' means to remove the first letter (without opening letter).
'prison' becomes 'jail' (jail can mean a prison).
'jail' with its first letter removed is 'ail'.
'n'+'ail'='NAIL'
(Other definitions for nail that I've seen before include "Hit this on the head to be bang-on" , "Metal pin, hit it on the head" , "Hit this on the head to be dead right" , "Fixing device" , "Amall metal spike" .)