Nasty crook with bad teeth caused trouble (6,3,4)
I believe the answer is:
rocked the boat
'caused trouble' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'nasty crook with bad teeth' is the wordplay.
'nasty' indicates an anagram (a nasty or wrong spelling).
'with' says to put letters next to each other.
'crook'+'bad'+'teeth'='crookbadteeth'
'crookbadteeth' with letters rearranged gives 'ROCKED THE BOAT'.
Can you help me to learn more?
(Another definition for rocked the boat that I've seen is " Created trouble in existing situation".)