A fool holding power can be extremely dangerous (4)
I believe the answer is:
asps
'be extremely dangerous' is the definition.
I can't tell whether this definition defines the answer.
'a fool holding power' is the wordplay.
'a fool' becomes 'ass' (both can mean a stupid person).
'holding' means one lot of letters goes inside another.
'power' becomes 'p' (symbol in physics).
'ass' going around 'p' is 'ASPS'.
'can' is the link.
Can you help me to learn more?
(Other definitions for asps that I've seen before include "native Africans?" , "Small vipers" , "Poisonous creatures" , "Pass around the snakes" , "Cleopatra's cobras" .)