Sat up eating father's light snacks (5)
I believe the answer is:
tapas
'light snacks' is the definition.
(tapas are Spanish snacks)
'sat up eating father's' is the wordplay.
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'eating' indicates putting letters inside.
'father' becomes 'pa' (pa can mean dad or father).
'sat' reversed gives 'tas'.
'tas' going around 'pa' is 'TAPAS'.
(Other definitions for tapas that I've seen before include "Spanish savoury dishes" , "Spanish eats" , "taste of Spain?" , "Savoury snacks" , "The Spanish scoff" .)