Sign of decay but not the first (5)
I believe the answer is:
aries
'sign' is the definition.
(star sign)
'decay but not the first' is the wordplay.
I cannot really see how this works, but
'first' could be 'i' (I in Roman numerals eg Elizabeth I) and 'i' is present in the answer.
The remaining letters 'ares' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'of' is the link.
Can you help me to learn more?
(Other definitions for aries that I've seen before include "Philip -, English poet" , "Ram star sign" , "House" , "Sheep" , "Stars" .)