Why?.....

[doorknob]

The cryptic clue is:
"Firm prepared rector to become saint"

6 Letters - S?E?D?

'Firm' would suggest STEADY is the answer but where does the rest of the clue fit in?

[scorpiojo]

Replace the r (Rector) in ready with S(ain)t

[scorpiojo]

...ready = prepared

[doorknob]

So easy when you put it like that, thanks, Scorpiojo.