Instrument, one in a lather, according to Spooner (6,4)
I believe the answer is:
double bass
'instrument' is the definition.
(double bass is a kind of instrument)
I can't explain the remainder of the clue.
(Other definitions for double bass that I've seen before include "One taking a bow" , "Musical instrument" , "Low-pitched string instrument" , "Stringed instrument" , "... as part of orchestra" .)