Small part of a potato masher (4)
I believe the answer is:
atom
'small part' is the definition.
(I've seen this in another clue)
'of a potato masher' is the wordplay.
'of' indicates a hidden word.
'ATOM' is hidden within 'potato masher'.
(Other definitions for atom that I've seen before include "An explosive particle as in bomb" , "Particle of matter now divided" , "Neutron and proton" , "Smallest particle in an element able to take part in a chemical reaction" , "Very small object or amount" .)