Brains are required for school principals (5)
I believe the answer is:
heads
This could be a double definition.
'brains' is the first definition.
(I know this)
'for school principals' is the second definition.
('head' can be a synonym of 'school principal')
'are required' acts as a link.
This may not be correct. Some or all of it may belong to one of the definitions.
Can you help me to learn more?
(Other definitions for heads that I've seen before include "a 50-50 chance" , "Obverse of a coin" , "Coin-tossing call" , "Chiefs" , "Goes for" .)