Cut into three parts (7)
I believe the answer is:
trisect
'cut into three parts' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for trisect that I've seen before include "cut into thirds" , "Cut into three parts" , "breakaway group after sound test" , "Cut into three pieces" , "Divide into three equal parts" .)