Pen pal finally stops once hurt, certainly (9)
I believe the answer is:
enclosure
'pen' is the definition.
(I've seen this before)
'pal finally stops once hurt certainly' is the wordplay.
'finally' suggests the final letters.
'stops' means one lot of letters goes inside another.
'hurt' indicates anagramming the letters.
'certainly' becomes 'sure' ('sure' can be a synonym of 'certainly'**).
The last letter of 'pal' is 'l'.
'once' with letters rearranged gives 'enco'.
'l' put inside 'enco' is 'enclo'.
'enclo'+'sure'='ENCLOSURE'
(Other definitions for enclosure that I've seen before include "Area fenced off - one's ulcer (anag)" , "Sealed-off area" , "It's included, with a letter say" , "pound" , "Pen" .)