One not merely pettish, but dangerous! (3,3)
I believe the answer is:
mad dog
'merely pettish but dangerous' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'one not' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found within the answer.
'not' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mad dog that I've seen before include "Transmitter of rabies?" , "Englishman's companion in the midday sun, according to Coward" .)