Two clues remaining which I cannot parse so explanations please.
a) Time to replace Charlie as lead in musical - that's strong stuff (7)
Cabaret (musical) becomes TABARET if you replace the C with a T but why do they mention "strong stuff"?
b) Looked for food for it, like grandpa (4) A?E?
I can only think of "aged". Are they playing with the word "foraged"? I'm not happy with "aged" as cannot parse it.