I am joining group of musicians - list is fixed (9)
I believe the answer is:
immutable
'is fixed' is the definition.
(thesaurus)
'i am joining group of musicians list' is the wordplay.
'i am' becomes 'I'm' (contraction).
'joining' says to put letters next to each other.
'group of musicians list' becomes 'mutable' (I can't explain this - if you can you should believe this answer much more).
'im'+'mutable'='IMMUTABLE'
Can you help me to learn more?
(Other definitions for immutable that I've seen before include "Changeless" , "Not able to be changed" .)