Dependable form of alloy (5)
I believe the answer is:
loyal
'dependable' is the definition.
(I've seen this before)
'form of alloy' is the wordplay.
'form of' indicates an anagram (the letters need to be in another 'form').
'alloy' anagrammed gives 'LOYAL'.
(Other definitions for loyal that I've seen before include "Steadfast and true" , "firm in solidarity" , "Dedicated" , "True to one's allegiance" , "Solid" .)