Copper interrupting dinner, say, with secretary — it's my fault (3,5)
I believe the answer is:
mea culpa
'it's my fault' is the definition.
(I've seen this before)
'copper interrupting dinner say with secretary' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'interrupting' means one lot of letters goes inside another.
'dinner say' becomes 'meal' (dinner is a kind of meal).
'with' is a charade indicator (letters next to each other).
'secretary' becomes 'pa' (I've seen this in other clues).
'cu' put within 'meal' is 'meacul'.
'meacul'+'pa'='MEA CULPA'
(Other definitions for mea culpa that I've seen before include "An acknowledgement of personal fault or error" , "Total admission of one's guilt (Latin)" , "'I am to blame (3,5)'" , "Sorry!" , "Old Roman admission of guilt" .)