Military accessory at work, requiring billions invested (5)
I believe the answer is:
busby
'military accessory' is the definition.
(type of military hat)
'work requiring billions invested' is the wordplay.
'work' becomes 'busy' (busying is a kind of working).
'requiring billions' becomes 'b' (I can't explain this - if you can you should give a lot more credence to this answer).
'invested' means one lot of letters goes inside another.
'busy' going around 'b' is 'BUSBY'.
'at' is the link.
Can you help me to learn more?
(Other definitions for busby that I've seen before include "Guardsman's helmet" , "Guardsman's fur hat" , "Military fur hat" , "Bearskin hat" , "Mil. bearskin (colloq.)" .)