A particular tone expressed by high-flying Americans beginning to lecture (5)
I believe the answer is:
nasal
'a particular tone' is the definition.
(having a nasal tone)
'high-flying americans beginning to lecture' is the wordplay.
'high-flying americans' becomes 'NASA' (NASA fly into space).
'beginning to' indicates taking the first letters.
The initial letter of 'lecture' is 'l'.
'nasa'+'l'='NASAL'
'expressed by' acts as a link.
(Other definitions for nasal that I've seen before include "Relating to a facial organ" , "Like a twang" , "Relating to the nose" , "Relating to one's hooter" , "piece of armour" .)