One day, start to eat and fill up (4)
I believe the answer is:
sate
'fill up' is the definition.
(I've seen this in another clue)
'one day start to eat' is the wordplay.
'one day' becomes 'sat' (abbreviation for Saturday).
'start to' suggests taking the first letters.
The first letter of 'eat' is 'e'.
'sat'+'e'='SATE'
'and' is the link.
(Other definitions for sate that I've seen before include "Indonesian food" , "Provide with more than enough" , "Stuff with teas" , "cloy" , "Satisfy to excess" .)