Crazy bandleader joining the forces (5)
I believe the answer is:
barmy
'crazy' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for barmy that I've seen before include "Mad; crazy" , "round the bend" , "Bonkers" , "Mad (slang)" , "like Basil, arguably?" .)