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