Popular tenor starts to ruin ostentatious musical opening (5)
I believe the answer is:
intro
'musical opening' is the definition.
(I've seen this before)
'popular tenor starts to ruin ostentatious' is the wordplay.
'popular' becomes 'in' ('in' can mean trendy).
'tenor' becomes 't'.
'starts to' indicates taking the first letters.
The initial letters of 'ruin ostentatious' is 'ro'.
'in'+'t'+'ro'='INTRO'
(Other definitions for intro that I've seen before include "Beginning (informal)" , "First bars of a musical composition" , "First part of a news story (abbr)" , "Launch" , "One starts" .)