Cunningly traded piece diminished in value (11)
I believe the answer is:
depreciated
'diminished in value' is the definition.
'depreciated' can be an answer for 'diminished' (depreciating is a kind of diminishing). I'm not certain of the 'in value' bit.
'cunningly traded piece' is the wordplay.
'cunningly' is an anagram indicator (a cunning or tricky form of the letters).
'traded'+'piece'='tradedpiece'
'tradedpiece' anagrammed gives 'DEPRECIATED'.
Can you help me to learn more?
(Other definitions for depreciated that I've seen before include "Lost value (over time)" , "written down" , "became less valuable" .)