Prune unruly thorns holding end of knife (7)
I believe the answer is:
shorten
'prune' is the definition.
(thesaurus)
'unruly thorns holding end of knife' is the wordplay.
'unruly' indicates anagramming the letters.
'holding' is an insertion indicator.
'end of' suggests the final letters.
The final letter of 'knife' is 'e'.
'thorns' is an anagram of 'shortn'.
'shortn' going around 'e' is 'SHORTEN'.
(Other definitions for shorten that I've seen before include "Abbreviate" , "Reduce in length" , "Prune" , "Abridge" , "Make less long" .)