Who, ditching wife, dated girl? I'm ashamed to hear it (4,5)
I believe the answer is:
home truth
'i'm ashamed to hear it' is the definition.
(an unpleasant or embarrassing truth)
'who ditching wife dated girl?' is the wordplay.
'ditching' suggests deleting specific letters.
'wife' becomes 'w'.
'dated' becomes 'met' (to date someone is to meet them romantically).
'girl?' becomes 'ruth' (girl's name).
'who' with 'w' taken out is 'ho'.
'ho'+'met'+'ruth'='HOME TRUTH'
(Other definitions for home truth that I've seen before include "Wounding mention of another's weakness" , "unwelcome news" , "unwelcome fact - with 24a" , "Pointed statement" , "Unpleasant fact person is made to realise about himself" .)