Phosphorus and toxic heavy metal (not thorium) found in ancient garment (7)

'garment' is the definition.
(pallium is a kind of garment)

'phosphorus and toxic heavy metal not thorium found in ancient' is the wordplay.
I cannot really see how this works, but
'phosphorus' could be 'p' (P is the chemical symbol for phosphorus) and 'p' is located in the answer.
'metal' could be 'li' (Li is an example) and 'li' is found within the answer.
The remaining letters 'alum' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).

Can you help me to learn more?