Order Dutch treats, accepting half of mums make the grade (3,3,7)
I believe the answer is:
cut the mustard
'make the grade' is the definition.
(I've seen this before)
'order dutch treats accepting half of mums' is the wordplay.
'order' is an anagram indicator.
'accepting' is an insertion indicator (inserted letters are accepted).
'half of' indicates taking half.
'dutch'+'treats'='dutchtreats'
'dutchtreats' anagrammed gives 'cutthestard'.
'mums' cut in half is 'mu'.
'cutthestard' going around 'mu' is 'CUT THE MUSTARD'.
(Other definitions for cut the mustard that I've seen before include "Perform to standard" , "Make the grade" , "Harvested brassica plant" , "pass inspection" , "do well enough" .)