Form hole, digger ultimately going in (5)
I believe the answer is:
carve
'form' is the definition.
(I know that carve is a more specific form of form)
'hole digger ultimately going in' is the wordplay.
'hole' becomes 'cave' (I've seen this before).
'digger ultimately' becomes 'r' (I can't justify this - if you can you should believe this answer much more).
'going in' means one lot of letters goes inside another.
'cave' enclosing 'r' is 'CARVE'.
Can you help me to learn more?
(Other definitions for carve that I've seen before include "Cut (meat, stone, etc)" , "Cut meat with sharp knife" , "fashion" , "produced by desire, with a shifting to the left" , "Slice (joint)" .)