Senior officer to fix promotion during last stand (9)
I believe the answer is:
brigadier
'senior officer' is the definition.
'fix promotion during last stand' is the wordplay.
'fix' becomes 'rig' (rigging is a kind of fixing).
'promotion' becomes 'ad' (ad is a kind of promotion).
'during' means one lot of letters goes inside another.
'last stand' becomes 'bier' (stand used in funerals).
'rig'+'ad'='rigad'
'rigad' placed within 'bier' is 'BRIGADIER'.
'to' acts as a link.
(Other definitions for brigadier that I've seen before include "High-ranking army officer" , "Army rank" , "Military leader" , "Rank above colonel" , "Air bridge over to the military officer" .)