Violent melee in front of National Trust unit (7)
I believe the answer is:
element
'unit' is the definition.
(thesaurus)
'violent melee in front of national trust' is the wordplay.
'violent' indicates anagramming the letters.
'in front of' says to put letters next to each other.
'national trust' becomes 'nt'.
'melee' is an anagram of 'eleme'.
'eleme'+'nt'='ELEMENT'
(Other definitions for element that I've seen before include "Basic constituent; aspect" , "He, say" , "Basic chemical unit" , "fire, perhaps, once" , "An abstract part of something" .)