Key point as we see it: King to be replaced by Catholic (5)
I believe the answer is:
focus
'key point' is the definition.
'focus' can be an answer for 'point' (focus is a kind of point). I am not certain of the 'key' bit.
'we see it king to be replaced by catholic' is the wordplay.
I cannot really see how this works, but
'we' could be 'us' and 'us' is found within the answer.
'see' could be 'c' (the word for the letter, according to Chambers) and 'c' is found within the answer.
This explanation may well be incorrect...
'as' acts as a link.
Can you help me to learn more?
(Other definitions for focus that I've seen before include "Centre of concentration" , "Bull's eye" , "Point at which light rays converge or diverge" , "Central point" , "Target" .)