Opening section of piano’s trill (7)
I believe the answer is:
nostril
'opening' is the definition.
(nostril is a kind of opening)
'section of piano's trill' is the wordplay.
'section of' indicates a hidden word.
'NOSTRIL' can be found hidden inside 'pianos trill'.
(Other definitions for nostril that I've seen before include "Nasal cavity" , "Bit of a breather" , "Nasal opening" , "Through which one would snort" , "Nose hole" .)