Idle brains (4)
I believe the answer is:
loaf
I believe this is a double definition.
'idle' is the first definition.
(loafing is a kind of idling)
'brains' is the second definition.
(I've seen this before)
(Other definitions for loaf that I've seen before include "bloomer, perhaps" , "Spend time idly, having slang head" , "loll" , "Half of one is better than none" , "Some bread; 10 (slang)" .)