There's nothing bigger in case containing plastic rag (7)
I believe the answer is:
largest
'there's nothing bigger' is the definition.
I can't tell whether this definition defines the answer.
'in case containing plastic rag' is the wordplay.
'in case' becomes 'lest' (I've seen this before).
'containing' is an insertion indicator.
'plastic' indicates an anagram (plastic can mean easily moulded).
'rag' is an anagram of 'arg'.
'lest' enclosing 'arg' is 'LARGEST'.
Can you help me to learn more?
(Other definitions for largest that I've seen before include "Extremely generous" , "Greatest in size" , "Superlatively extensive" , "Biggest" , "Of greatest magnitude" .)