Destroying it, crest cut into three parts (7)
I believe the answer is:
trisect
'cut into three parts' is the definition.
(I've seen this before)
'destroying it crest' is the wordplay.
'destroying' is an anagram indicator.
'it'+'crest'='itcrest'
'itcrest' is an anagram of 'TRISECT'.
(Other definitions for trisect that I've seen before include "Divide into three equal parts" , "breakaway group after sound test" , "Cut into three parts" , "cut into thirds" , "Cut into three pieces" .)