Sort of pool-ball sound? (5)
I believe the answer is:
solid
'sound?' is the definition.
(similar in meaning)
'sort of pool-ball' is the wordplay.
I cannot quite see how this works, but
'of' could be 'o' and 'o' is located in the answer.
The remaining letters 'slid' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for solid that I've seen before include "Sound; substantial" , "Firm - sound - reliable" , "Weighty" , "Hard; three dimensional" , "Firm; not liquid" .)