Unreal, like the root of a negative number (9)
I believe the answer is:
imaginary
'unreal' is the definition.
(similar in meaning)
'like the root of a negative number' is the wordplay.
I cannot really see how this works, but
'like' could be 'y' (similar in meaning) and 'y' is found within the answer.
'a' is present in the answer.
'number' could be 'm' (M is an example) and 'm' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for imaginary that I've seen before include "Made up" , "Not real" , "Not based on fact, unreal" , "Illusory" , "Fanciful, notional" .)