Meal a fathead eats unwisely (5)
I believe the answer is:
feast
'meal' is the definition.
(I know that feast is a type of meal)
'a fathead eats unwisely' is the wordplay.
'a fathead' becomes 'f' (head letter of 'fat').
'unwisely' indicates anagramming the letters.
'eats' is an anagram of 'east'.
'f'+'east'='FEAST'
(Other definitions for feast that I've seen before include "Tuck-in; blow-out" , "Repast" , "Huge nosh-up" , "Abundant meal" , "Sumptuous meal" .)