Active when acquiring British military headgear (5)
I believe the answer is:
busby
'military headgear' is the definition.
'busby' can be an answer for 'headgear' (busby is a kind of headgear). I'm not certain of the 'military' bit.
'active when acquiring british' is the wordplay.
'active' becomes 'busy' (similar in meaning).
'when acquiring' indicates putting letters inside.
'british' becomes 'b' (abbreviation e.g. in BBC).
'busy' going around 'b' is 'BUSBY'.
Can you help me to learn more?
(Other definitions for busby that I've seen before include "Guardsman's fur hat" , "Bearskin cap (colloq.)" , "Military fur hat" , "Bearskin hat" , "Guardsman's helmet" .)