High-sounding buck returns unknown (5)
I believe the answer is:
reedy
'high-sounding' is the definition.
(as in a reedy voice)
'buck returns unknown' is the wordplay.
'buck' becomes 'deer' (buck can mean a male deer).
'returns' says the letters should be written backwards.
'unknown' becomes 'y' (algebra).
'deer' backwards is 'reed'.
'reed'+'y'='REEDY'
(Other definitions for reedy that I've seen before include "That'll sound a bit thin" , "Sounding like eg oboe" , "Shrill, or piping" , "Like oboe's sound?" , "Thin and piping" .)