Military subdivision (8)
(Other definitions for regiment that I've seen before include "The Grenadier Guards, for example" , "Large army unit" , "Army grouping" , "Metering (anag) - army unit" , "Section of an army" .)
'military subdivision' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for unit that I've seen before include "Single element; one" , "One part of a whole" , "Single manufactured item; military grouping" , "Individual thing as part of a whole" , "Individual thing; one" .)