Pretty good (5)
(Other definitions for solid that I've seen before include "Shape with three dimensions" , "cone-shaped, say" , "Idols (anag.)" , "Three-dimensional - compact" , "Neither liquid nor gas" .)
(Other definitions for not bad that I've seen before include "Acceptable" , "Negative sort of praise, fairly good" , "Passable" , "So-so" , "Quite good" .)