Bearing one in pieces (4)
I believe the answer is:
mien
'bearing' is the definition.
('mien' can be a synonym of 'bearing')
'one in pieces' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' is an insertion indicator.
'pieces' becomes 'men' ('man' can be a synonym of 'piece').
'i' going into 'men' is 'MIEN'.
(Other definitions for mien that I've seen before include "Bearing, demeanour" , "A face like mine" , "person's look" , "aspect" , "Personal bearing" .)