TSQL - functions..

yes i want to declare it and call it in the stored procedure.

For the record : declaring a function in dynamic sql and using it in dynamic sql does not make your code more readable.

We don't have a clue on what that function has to do so we can't suggest on how to do it another way.
For example
I think #tables or table variables is a way to deal with 'variable circomstances' in a process when it comes to which list of values applie on this moment.

Actually, if you look more closely, you'll see that I was careful enough to say *RE*create a function.  The function would need to already exist, at least as a shell, but could then be replaced by code generated dynamically.

Yes, you would need to do your own serialization in that case, but that's not terribly difficult, viz sp_getapplock & sp_releaseapplock.