Regret referring to change in mores (7)
I believe the answer is:
remorse
'regret' is the definition.
(remorse is a feeling of regret)
'referring to change in mores' is the wordplay.
'referring to' becomes 're' ('re' means regarding or about).
'change in' is an anagram indicator.
'mores' with letters rearranged gives 'morse'.
're'+'morse'='REMORSE'
(Other definitions for remorse that I've seen before include "Feeling of guilt" , "Penitence" , "it may bite" , "Distress from a sense of guilt" , "Self-reproach" .)