Secretive, after spilling cereals without paying attention (10)
I believe the answer is:
carelessly
'without paying attention' is the definition.
(done without proper care)
'secretive after spilling cereals' is the wordplay.
'secretive' becomes 'sly' (I've seen this in other clues).
'after' means one lot of letters go next to another.
'spilling' is an anagram indicator.
'cereals' is an anagram of 'careles'.
'sly' put after 'careles' is 'CARELESSLY'.
(Other definitions for carelessly that I've seen before include "Casually" , "without proper attention" , "Without concern" , "Incautiously" , "sloppily" .)