Poor cast includes three from Swedish group (6)
I believe the answer is:
shabby
'poor' is the definition.
(shabby can mean of poor quality)
'cast includes three from swedish group' is the wordplay.
'cast' becomes 'shy' (both can mean to throw or fling).
'includes' indicates putting letters inside.
'three from' means to remove the last letter (three of the four letters).
'swedish group' becomes 'abba' (Swedish band).
'abba' with its final letter taken off is 'abb'.
'shy' going around 'abb' is 'SHABBY'.
(Other definitions for shabby that I've seen before include "Ill-dressed - mean" , "Down at heel" , "Threadbare - dingy - mean - base" , "Worn and dirty; mean" , "Old and threadbare" .)