Boy sent back hospital food (4)
I believe the answer is:
nosh
'food' is the definition.
(nosh is a kind of food)
'boy sent back hospital' is the wordplay.
'boy' becomes 'son' (synonyms).
'sent back' says the letters should be written backwards.
'hospital' becomes 'H' (abbreviation for hospital).
'son' backwards is 'nos'.
'nos'+'h'='NOSH'
(Other definitions for nosh that I've seen before include "Eat (sl.)" , "'Food, grub (4)'" , "Food (sl.)" , "Food (Yiddish)" , "Food - eat greedily" .)