Lad that's served up hot food (4)
I believe the answer is:
nosh
'food' is the definition.
(nosh is a kind of food)
'lad that's served up hot' is the wordplay.
'lad' becomes 'son'.
'that's served up' says the letters should be written backwards (in a down clue up is reversing the order).
'hot' becomes 'h'.
'son' backwards is 'nos'.
'nos'+'h'='NOSH'
(Other definitions for nosh that I've seen before include "(Eat) a snack" , "Eats" , "Eat (sl.)" , "eating this?" , "Slangy food or meal" .)