Stops oatmeal getting sprinkled around large trifle (5,8)
I believe the answer is:
small potatoes
'trifle' is the definition.
(I've seen this in another clue)
'stops oatmeal getting sprinkled around large' is the wordplay.
'getting sprinkled' indicates anagramming the letters (the letters are sprinkled around).
'around' means one lot of letters goes inside another.
'large' becomes 'L' (eg in clothes sizes).
'stops'+'oatmeal'='stopsoatmeal'
'stopsoatmeal' anagrammed gives 'smalpotatoes'.
'smalpotatoes' going around 'l' is 'SMALL POTATOES'.
(Other definitions for small potatoes that I've seen before include "piddling" , "insignifant amount" , "A greengrocer might sell them" , "Person or items of little worth" .)