Bit in potato mash (4)
I believe the answer is:
atom
'bit' is the definition.
(I have seen 'Tiny bit' mean 'atom' so perhaps 'bit' could also mean 'atom')
'in potato mash' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ATOM' can be found hidden inside 'potato mash'.
(Other definitions for atom that I've seen before include "It was once thought to be smallest particle of matter" , "Elementary particle" , "Small amount of matter" , "A wee bit" , "Tiny particle, great bomb" .)