I'm sorry copper in restaurant rendezvous with secretary (3,5)
I believe the answer is:
mea culpa
'i'm sorry' is the definition.
'copper in restaurant rendezvous with secretary' is the wordplay.
'copper' becomes 'cu' (chemical symbol).
'in' is an insertion indicator.
'restaurant rendezvous' becomes 'meal'.
'with' is a charade indicator (letters next to each other).
'secretary' becomes 'pa' (I've seen this in other clues).
'cu' placed into 'meal' is 'meacul'.
'meacul'+'pa'='MEA CULPA'
(Other definitions for mea culpa that I've seen before include "'I am to blame (3,5)'" , "An acknowledgement of personal fault or error" , "I'm guilty (Latin)" , "Old Roman admission of guilt" , "Acknowledgment of error or guilt" .)