One in troops bearing (4)
I believe the answer is:
mien
'bearing' is the definition.
(I know that bearing can be written as mien)
'one in troops' is the wordplay.
'one' becomes 'I' (Roman numeral).
'in' means one lot of letters goes inside another.
'troops' becomes 'men' (I've seen this in another clue).
'i' going into 'men' is 'MIEN'.
(Other definitions for mien that I've seen before include "Person's bearing" , "Dignified manner" , "Personal bearing" , "Comportment" , "A person's demeanour" .)