A setback for Aries that will be detrimental (3)
I believe the answer is:
mar
'aries that will be detrimental' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'a setback' is the wordplay.
I cannot quite see how this works, but
'a' is within the answer.
The remaining letters 'mr' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'for' acts as a link.
Can you help me to learn more?
(Other definitions for mar that I've seen before include "Spoil stuff" , "Ruin" , "Detract from" , "wreck" , "Spoil something" .)