A fake prophecy: 'Earth's lost metal' — it's doubtful (10)
I believe the answer is:
apocryphal
'doubtful' is the definition.
(thesaurus)
'a fake prophecy earth's lost metal' is the wordplay.
'fake' is an anagram indicator (I've seen this in other clues).
'earth' becomes 'E'.
'lost' suggests deleting specific letters.
'metal' becomes 'Al' (chemical symbol for aluminium).
'prophecy' with 'e' taken out is 'prophcy'.
'prophcy' with letters rearranged gives 'pocryph'.
'a'+'pocryph'+'al'='APOCRYPHAL'
'it's' acts as a link.
(Other definitions for apocryphal that I've seen before include "Fabulous" , "probably made-up" .)