Having ruled the novice out, wished one hadn't (4)
I believe the answer is:
rued
'wished one hadn't' is the definition.
(I've seen this in another clue)
'having ruled the novice out' is the wordplay.
'having' becomes 'ru' (I am not sure about this - if you are sure you should believe this answer much more).
'ruled' becomes 'led' (I've seen this in other clues).
'the novice' becomes 'L' (as in a learner driver with L-plates on their car).
'out' is a deletion indicator.
'led' with 'l' taken away is 'ed'.
'ru'+'ed'='RUED'
Can you help me to learn more?
(Other definitions for rued that I've seen before include "was remorseful" , "'Regretted, repented (4)'" , "Was contrite about" , "Was sorry" , "Bitterly regretted" .)