Formula works but not when part of the main formula?

Assuming that the logic of your formula is correct, you need to put the "built" range addresses inside the INDIRECT function.

=INDEX(INDIRECT("Stock!"&ADDRESS(MATCH(LEFT(T5,FIND(":",T5,1)-1),Stock!C:C,0),11)&":"&ADDRESS(MATCH("Total "&LEFT(T5,FIND(":",T5,1)-1),Stock!C:C,0),11)),MATCH(LEFT(D5,FIND(" ",D5,1)-1),INDIRECT("Stock!"&ADDRESS(MATCH(LEFT(T5,FIND(":",T5,1)-1),Stock!C:C,0),4)&":"&ADDRESS(MATCH("Total "&LEFT(T5,FIND(":",T5,1)-1),Stock!C:C,0),4)),1))