Cracking nut, a scarce thing with a shell? (10)
I believe the answer is:
crustacean
'thing with a shell?' is the definition.
(crustaceans have shells)
'cracking nut a scarce' is the wordplay.
'cracking' indicates anagramming the letters.
'nut'+'a'+'scarce'='nutascarce'
'nutascarce' is an anagram of 'CRUSTACEAN'.
(Other definitions for crustacean that I've seen before include "Animal like a crab" , "Hard-shelled animal" , "life form with hard case" , "seafood" , "Lobster perhaps" .)