Preserve in aspic a nut (3)
I believe the answer is:
can
'preserve' is the definition.
(canning is a kind of preserving)
'in aspic a nut' is the wordplay.
'in' indicates the answer is hidden within the clue.
'CAN' is hidden in the letters of 'aspic a nut'.
(Other definitions for can that I've seen before include "Preserve fruit" , "Vessel" , "Is able; container" , "Possesses ability" , "Metal container for food" .)