Opening one, torn apart (5)
I believe the answer is:
intro
'opening' is the definition.
(short for introduction)
'one torn apart' is the wordplay.
'one' becomes 'i' (Roman numeral).
'apart' indicates anagramming the letters.
'torn' is an anagram of 'ntro'.
'i'+'ntro'='INTRO'
(Other definitions for intro that I've seen before include "Launch" , "Opener" , "overture" , "Beginning passage" , "One starts" .)