Catherine always had a spare part for the Rolls (4)
I believe the answer is:
parr
'catherine always' is the definition.
I can't judge whether this definition defines the answer.
'a spare part for the rolls' is the wordplay.
'a spare part' becomes 'pa' (I can't justify this - if you can you should give a lot more credence to this answer).
'for' means one lot of letters go next to another.
'the rolls' becomes 'RR' (Rolls Royce).
'pa'+'rr'='PARR'
'had' acts as a link.
Can you help me to learn more?
(Other definitions for parr that I've seen before include "Catherine --, last wife of Henry VIII" , "Katherine --, last wife of Henry VIII" , "Wife who survived" , "Young salmon or trout" , "Young salmon - wife of Henry VIII" .)