Finding silver in mud is an optical illusion (6)
I believe the answer is:
mirage
'optical illusion' is the definition.
(mirage is a kind of optical illusion)
'finding silver in mud is an' is the wordplay.
I cannot quite see how this works, but
'silver' could be 'ag' (Ag is the chemical symbol for silver) and 'ag' is present in the answer.
'mud' could be 'mire' (synonyms) and 'mire' is present in the remaining letters.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for mirage that I've seen before include "Optical illusion, in desert perhaps" , "Illusory sighting" , "Hallucination caused by hot air" , "illusory thing" , "Illusion caused by atmospheric refraction" .)