Constant change affecting most fads and tastes (9)
'constant' is the definition.
(similar in meaning)
'change affecting most fads and tastes' is the wordplay.
I cannot really understand how this works, but
an anagram of 'fads' is 'dfas' which is present in the answer.
This may be the basis of clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for steadfast that I've seen before include "Resolutely firm", "Firm, fixed and faithful", "Resolute, unwavering", "Constant, not changing or yielding", "Strong".)