Modified Bronx diet, one with lighter content (9)
I believe the answer is:
tinderbox
'one with lighter content' is the definition.
(a tinderbox contains stuff for lighting a fire)
'modified bronx diet' is the wordplay.
'modified' indicates an anagram.
'bronx'+'diet'='bronxdiet'
'bronxdiet' anagrammed gives 'TINDERBOX'.
(Other definitions for tinderbox that I've seen before include "Old container for fire-lighting equipment" , "explosive type?" , "Potentially explosive situation" , "Old lighter" , "Volatile person" .)