Bird spends half its life cracking nut (7)
I believe the answer is:
pelican
'bird' is the definition.
(pelican is a kind of bird)
'life cracking nut' is the wordplay.
'life' becomes 'li' (I can't explain this - if you can you should give a lot more credence to this answer).
'cracking' indicates putting letters inside (inserted letters go into a gap or crack).
'nut' becomes 'pecan' (type of nut).
'li' going inside 'pecan' is 'PELICAN'.
'spends half its' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for pelican that I've seen before include "Bird with large pouched beak" , "one with huge bill" , "Large bird with pouch in bill for storing fish" , "Large-billed waterbird" , "Bird, large bag under bill" .)