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' is an anagram indicator.
'alloy' anagrammed gives 'LOYAL'.
'that's' is the 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" .)