Shame's mine and yours, but not ours (4)
I believe the answer is:
pity
'shame's' is the definition.
('pity' can be a synonym of 'shame')
'mine and yours but not ours' is the wordplay.
'mine' becomes 'pit' (down the pit**).
'and' says to put letters next to each other.
'but not' indicates named letters should be taken away.
'yours' with 'ours' taken out is 'y'.
'pit'+'y'='PITY'
(Other definitions for pity that I've seen before include "Commiseration" , "Have sympathetic feelings toward" , "Ruth" , "Feel compassion for" , "Cause of disappointment" .)