Brain left fool (4)
I believe the answer is:
loaf
'brain' is the definition.
(I've seen this before)
'left fool' is the wordplay.
'left' becomes 'l' (common abbreviation).
'fool' becomes 'oaf' (I've seen this before).
'l'+'oaf'='LOAF'
(Other definitions for loaf that I've seen before include "BRINK/BRICK" , "Lounge about" , "bloomer, say" , "Pass time idly" , "Bread - brain!" .)