Catherine's dad has 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 cannot understand how one could define the other.
'dad has a posh car' is the wordplay.
'dad' becomes 'pa' (synonyms).
'has' is a charade indicator (letters next to each other).
'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 or trout" , "Katherine --, last wife of Henry VIII" , "Catherine --, Henry VIII's sixth wife" , "Young salmon; Queen Catherine ---" , "Fish" .)