Invalid possessing no legal power (4)
I believe the answer is:
null
'invalid' is the definition.
(similar in meaning)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for null that I've seen before include "Without force, validity" , "Legally void" , "Useless - nothing" , "Without value or consequence" , "Invalid, void" .)