Gravedigger Mark’s back carrying cross (6)
I believe the answer is:
sexton
'gravedigger' is the definition.
(I've seen this before)
'mark's back carrying cross' is the wordplay.
'mark's back' becomes 'seton' (I can't justify this - if you can you should give a lot more credence to this answer).
'carrying' means one lot of letters goes inside another.
'cross' becomes 'x' (X is cross-shaped).
'seton' enclosing 'x' is 'SEXTON'.
Can you help me to learn more?
(Other definitions for sexton that I've seen before include "Church bell ringer and gravedigger" , "Church bell-ringer or grave-digger" , "Person who looks after church and churchyard" , "Church caretaker, gravedigger" , "Grave digger" .)