True transmutation of alloy (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this before)
'transmutation of alloy' is the wordplay.
'transmutation of' indicates anagramming the letters (I've seen 'transmutation' mean this).
'alloy' with letters rearranged gives 'LOYAL'.
(Other definitions for loyal that I've seen before include "Keeping faith" , "Sort of toast" , "Dedicated" , "Steadfast and true" , "Faithful - reliable" .)