Catherine's dad, a bit of a terror (4)
I believe the answer is:
parr
'catherine's' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'dad a bit of a terror' is the wordplay.
'dad' becomes 'pa' (both can mean one's father).
'a bit of a terror' becomes 'rr' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'pa'+'rr'='PARR'
Can you help me to learn more?
(Other definitions for parr that I've seen before include "Wife who survived" , "survivor of the decapitator?" , "Young salmon with spots" , "Young salmon - wife of Henry VIII" , "Young salmon; Queen Catherine ---" .)