Last of five cracks finished off nut (4)
I believe the answer is:
head
'nut' is the definition.
(nut can mean one's head)
'last of five cracks finished' is the wordplay.
'last of' says to take the final letters.
'cracks' indicates putting letters inside (crack their way in).
'finished' becomes 'had' (I can't explain this - if you can you should believe this answer much more).
The final letter of 'five' is 'e'.
'e' going inside 'had' is 'HEAD'.
'off' is the link.
Can you help me to learn more?
(Other definitions for head that I've seen before include "Chief; bonce" , "Source of river, leader" , "School boss" , "Skull" , "River or stream source" .)