High and thin in tone (5)
I believe the answer is:
reedy
'high and thin in tone' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reedy that I've seen before include "full of vegetation" , "edge of some river-banks" , "(Of sound) high and thin" , "(Of sound) thin in tone" , "Shrill, or piping" .)