Officer material with a unit, periodically (8)
I believe the answer is:
sergeant
'officer' is the definition.
(I know that Sergeant is a type of Japanese police officer)
'material with a unit periodically' is the wordplay.
'material' becomes 'serge' (serge is a kind of material).
'with' is a charade indicator (letters next to each other).
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternate letters of 'unit' are 'nt'.
'serge'+'a'+'nt'='SERGEANT'
(Other definitions for sergeant that I've seen before include "Police rank" , "Pepper, say" , "Noncommissioned military officer" , "NCO; Death a fell one, Hamlet" , "Soldier and lover in Hardy novel" .)