Chap’s moral philosophy greatly diminished (5)
I believe the answer is:
ralph
'chap's' is the definition.
'moral philosophy greatly diminished' is the wordplay.
'greatly diminished' says the answer is hidden in the clue (diminish the letters to reveal the hidden word).
'RALPH' is hidden within 'moral philosophy'.
(Other definitions for ralph that I've seen before include "Boy's name" , "Man's name" , "___ Fiennes, English actor, b. 1962" , "- Vaughan Williams, English composer" , "--- Waldo Emerson, US writer" .)