Steel is terribly loyal (5)
I believe the answer is:
alloy
'steel' is the definition.
(alloy steel is a kind of steel)
'terribly loyal' is the wordplay.
'terribly' indicates an anagram.
'loyal' with letters rearranged gives 'ALLOY'.
'is' acts as a link.
(Other definitions for alloy that I've seen before include "temper" , "A mix of two or more metals" , "Metals fused together" , "Metal mixture" , "Bronze or brass, perhaps" .)