Part of generator seasoned soldiers set up first (8)
I believe the answer is:
armature
'part of generator' is the definition.
(rotating part of a generator)
'seasoned soldiers set up first' is the wordplay.
'seasoned' becomes 'mature' (seasoned can mean mature or old).
'soldiers' becomes 'RA' (abbreviation for Royal Artillery).
'set up' says the letters should be written in reverse (in a down clue, letters go up).
'first' says to put letters next to each other.
'ra' back-to-front is 'ar'.
'mature' after 'ar' is 'ARMATURE'.
(Other definitions for armature that I've seen before include "Rotating dynamo coil - a rum tear (anag)" , "Part of dynamo" , "keeper" , "Rotating coil of an electric motor" , "equipment for defence" .)