Sparkling start to gala - rubbish by end of day (8)
I believe the answer is:
glittery
'sparkling' is the definition.
('glittery' can be a synonym of 'sparkling')
'start to gala rubbish by end of day' is the wordplay.
'start to' indicates taking the first letters.
'rubbish' becomes 'litter' (litter is a kind of rubbish**).
'by' means one lot of letters go next to another.
'end of' indicates one should take the final letters.
The first letter of 'gala' is 'g'.
The final letter of 'day' is 'y'.
'g'+'litter'+'y'='GLITTERY'
(Other definitions for glittery that I've seen before include "snazzy" , "Sparkly" , "Brightly shining" .)