One torn apart in opening (5)
I believe the answer is:
intro
'opening' is the definition.
(short for introduction)
'one torn apart in' is the wordplay.
'one' becomes 'i' (Roman numeral).
'apart' indicates anagramming the letters (the word is broken apart).
'in' is an insertion indicator.
'torn' with letters rearranged gives 'ntro'.
'i' placed inside 'ntro' is 'INTRO'.
(Other definitions for intro that I've seen before include "Preliminary part" , "One starts" , "Musical opening" , "Prologue, opening (abbr.)" , "Nitro (anag.)" .)