Bit of food? Child eats seconds with a mother going short (7)
I believe the answer is:
tostada
'bit of food?' is the definition.
(type of Mexican dish)
'child eats seconds with a mother going short' is the wordplay.
'child' becomes 'tot' (term for a young child).
'eats' indicates putting letters inside.
'seconds' becomes 's'.
'with' means one lot of letters go next to another.
'a mother going short' becomes 'ada' (I can't explain this - if you can you should believe this answer much more).
'tot' going around 's' is 'tost'.
'tost'+'ada'='TOSTADA'
(Other definitions for tostada that I've seen before include "Mexican topped pancake" , "Mexican food" , "Crisp 15 down" , "spicy food" .)