Silver in mud? An illusion (6)
I believe the answer is:
mirage
'an illusion' is the definition.
(I've seen this in another clue)
'silver in mud?' is the wordplay.
'silver' becomes 'Ag' (Ag is the chemical symbol for silver).
'in' indicates putting letters inside.
'mud?' becomes 'mire' ('mire' can be a synonym of 'mud').
'ag' put within 'mire' is 'MIRAGE'.
(Other definitions for mirage that I've seen before include "vision" , "See it to believe it?" , "It doesn't exist" , "Deceptive image, in the desert say" , "Illusory image" .)