The sort of alloy that's true (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this before)
'the sort of alloy' is the wordplay.
'the sort of' is an anagram indicator.
'alloy' with letters rearranged gives 'LOYAL'.
'that's' acts as a link.
(Other definitions for loyal that I've seen before include "Showing firm allegiance" , "True to one's country etc" , "Faithful - true" , "Alloy (anag.)" , "Trustworthy" .)