Error mine, ultimately? That's rich! (5)
I believe the answer is:
torte
'that's rich' is the definition.
(rich cake)
'error mine ultimately?' is the wordplay.
'error' becomes 'tort' (legal term for a type of wrongdoing).
'ultimately?' suggests the final letters (I've seen 'ultimately' mean this).
The last letter of 'mine' is 'e'.
'tort'+'e'='TORTE'
(Other definitions for torte that I've seen before include "Rich food" , "Sweet cake or tart" , "Rich sweet cake" , "(Austrian) cake with cream and fruit or nuts" , "German cream cake" .)