Discourteous when daughter leaves leading to regret (3)
I believe the answer is:
rue
'regret' is the definition.
(synonyms)
'discourteous when' is the wordplay.
'discourteous' becomes 'ru' (I am not sure about this - if you are sure you should believe this answer much more).
'when' becomes 'e' (I've seen this in other clues).
'ru'+'e'='RUE'
'daughter leaves leading to' is the link.
This may not be correct. Some or all of it may belong to another bit of the clue.
(Other definitions for rue that I've seen before include "Swann's way?" , "repentance" , "Be regretful" , "Regret; road (Fr.)" , "Wish not to have been" .)