American preserves tiny egg parts (5)
I believe the answer is:
units
'parts' is the definition.
(unit is a kind of part)
'american preserves tiny egg' is the wordplay.
'american' becomes 'us'.
'preserves' indicates putting letters inside.
'tiny egg' becomes 'nit'.
'us' enclosing 'nit' is 'UNITS'.
(Other definitions for units that I've seen before include "Apparatus similar to barbecue" , "Single things" , "Shops in a mall, for example" , "Individual things as parts of a whole" , "Single parts of a whole" .)