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 present in the answer.
The remaining letters 'alum' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

Can you help me to learn more?