Full day drained egghead (5)
I believe the answer is:
sated
'full' is the definition.
(I've seen this in another clue)
'day drained egghead' is the wordplay.
'day' becomes 'sat' (abbreviation for Saturday).
'drained' says to hollow out the word (remove centre letters).
'egghead' with its centre taken out is 'ed'.
'sat'+'ed'='SATED'
(Other definitions for sated that I've seen before include "Stuffed with food, too full" , "completely filled?" , "with no empty spaces?" , "Fully satisfied" , "Fed up" .)