Catherine's dad had a posh car (4)
I believe the answer is:
parr
'catherine's' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'dad had a posh car' is the wordplay.
'dad' becomes 'pa' (pa can mean someone's father).
'had' means one lot of letters go next to another.
'a posh car' becomes 'rr' (short for Rolls Royce).
'pa'+'rr'='PARR'
Can you help me to learn more?
(Other definitions for parr that I've seen before include "Young salmon with spots" , "Wife surviving" , "Henry VIII's sixth queen" , "Katherine --, last wife of Henry VIII" , "Catherine ---, Henry VIII's widow" .)