Choose not to take part (3,3)
I believe the answer is:
opt out
'choose not to take part' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for opt out that I've seen before include "'Choose not to take part (3,3)'" , "Choose to be excluded" , "Choose not to participate" , "Choose not to do something" , "Decide not to participate" .)