Fantastic claim to work with silver (7)
I believe the answer is:
magical
'fantastic' is the definition.
(fantastic can mean magical or imaginary)
'claim to work with silver' is the wordplay.
'to work' indicates anagramming the letters (letters worked into a new order).
'with' indicates putting letters inside.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'claim' with letters rearranged gives 'mical'.
'mical' going around 'ag' is 'MAGICAL'.
(Other definitions for magical that I've seen before include "tricky?" , "Charming" , "Fascinating - occult" , "Enchanting or spell-binding" , "charmed" .)