Can't raise rumpus or pin it down (9)
I believe the answer is:
ascertain
'pin it down' is the definition.
I can't judge whether this definition defines the answer.
'can't raise rumpus' is the wordplay.
'rumpus' indicates an anagram (rumpus can mean a disturbance).
'cant'+'raise'='cantraise'
'cantraise' with letters rearranged gives 'ASCERTAIN'.
'or' acts as a link.
Can you help me to learn more?
(Other definitions for ascertain that I've seen before include "Establish after an investigation or study" , "Establish (the facts)" , "Find out for sure, determine" , "Verify" , "Establish, find out" .)