Kind of pigeon that's prepared to sing (5)
I believe the answer is:
'kind of pigeon' is the definition.
(a stool pigeon is a police informer)
'prepared to sing' is the wordplay.
I cannot quite understand how this works, but
'to' is found in the answer.
The remaining letters 'sol' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'that's' is the link.
Can you help me to learn more?
(Other definitions for stool that I've seen before include "Motion" , "Armless seat" , "place butt on one?" , "Simple seat" , "Three-legged backless seat" .)