Lad sent back hospital grub (4)
I believe the answer is:
nosh
'grub' is the definition.
(I've seen this before)
'lad sent back hospital' is the wordplay.
'lad' becomes 'son'.
'sent back' says the letters should be written backwards.
'hospital' becomes 'h'.
'son' written backwards gives 'nos'.
'nos'+'h'='NOSH'
(Other definitions for nosh that I've seen before include "Food (Yiddish)" , "Food (coll)" , "A slang meal" , "'Food, grub (4)'" , "Scoff" .)