After short time, quietly an open introduction to a set of kettledrums (7)
I believe the answer is:
timpani
'a set of kettledrums' is the definition.
(I've seen this in another clue)
'after short time quietly an open introduction' is the wordplay.
'after' is a charade indicator (letters next to each other).
'short' means to remove the last letter.
'quietly' becomes 'p' (p for piano and quiet).
'open' indicates taking the first letters (take the opening letter).
The first letter of 'introduction' is 'i'.
'time' with its final letter removed is 'tim'.
'tim' next to 'p' is 'timp'.
'timp'+'an'+'i'='TIMPANI'
'to' acts as a link.
(Other definitions for timpani that I've seen before include "Percussion instrument" , "Percussion section" , "Mini tap on a large drum" , "> kitchen equipment?" , "PM, I ain't (anag.)" .)