Who, ditching wife, dated girl — one's ashamed to hear it (4,5)
I believe the answer is:
home truth
'one's ashamed to hear it' is the definition.
(an unpleasant or embarrassing truth)
'who ditching wife dated girl' is the wordplay.
'ditching' is a deletion indicator.
'wife' becomes 'w'.
'dated' becomes 'met' (to date someone is to meet them romantically).
'girl' becomes 'ruth' (girl's name).
'who' with 'w' taken away 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 fact - with 24a" , "unwelcome news" , "unwelcome remark" , "Unpleasant fact told to someone about themselves" .)