Regret breaking centre pane (10)
I believe the answer is:
repentance
'regret' is the definition.
(I know that repentance is a type of remorse)
'breaking centre pane' is the wordplay.
'breaking' is an anagram indicator (back).
'centre'+'pane'='centrepane'
'centrepane' anagrammed gives 'REPENTANCE'.
(Other definitions for repentance that I've seen before include "Sincere remorse" , "Contrition" , "Regret, remorse" , "Feeling of remorse" .)