Brain food (4)
I believe the answer is:
loaf
I believe this clue is a double definition.
'brain' is the first definition.
(I've seen this before)
'food' is the second definition.
(loaf is a kind of food)
(Other definitions for loaf that I've seen before include "Spend time idly, having slangy head" , "Mass of bread" , "Some bread; 10 (slang)" , "Bread - brain!" , "Bum about" .)