Sort of alloy that's true (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this before)
'sort of alloy' is the wordplay.
'sort of' indicates anagramming the letters.
'alloy' anagrammed gives 'LOYAL'.
'that's' acts as a link.
(Other definitions for loyal that I've seen before include "Keeping faith" , "Faithful - true" , "Faithful to country, cause or friend" , "Trusty" , "Steadfast in allegiance" .)