'Customers,' I plea, 'must get special treatment. They are very valuable!' (8,6)

I believe the answer is:

precious metals

'they are very valuable' is the definition.
(precious metals are valuable)

'customers i plea must get special treatment' is the wordplay.
'must get special treatment' indicates anagramming the letters (letters treated or processed differently).
'customers'+'i'+'plea'='customersiplea'
'customersiplea' is an anagram of 'PRECIOUS METALS'.

I've seen this clue in The Telegraph.

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