The gentleman who cut father short (3)
I believe the answer is:
sir
'the gentleman who cut' is the definition.
'sir' can be an answer for 'the gentleman' (a man may be addressed as sir). I am unsure of the rest of the definition.
'father short' is the wordplay.
'father' becomes 'sire' (synonyms).
'short' means to remove the last letter.
'sire' with its last letter removed is 'SIR'.
Can you help me to learn more?
(Other definitions for sir that I've seen before include "Polite form of address to a man" , "Title of knight or baronet" , "Form of address for a knight" , "Knight's prefix" , "Polite or formal address to male" .)