Paul came over. It's my fault! (3,5)
I believe the answer is:
mea culpa
'it's my fault' is the definition.
(Latin phrase meaning 'by my fault')
'paul came over' is the wordplay.
'over' indicates anagramming the letters.
'paul'+'came'='paulcame'
'paulcame' is an anagram of 'MEA CULPA'.
(Other definitions for mea culpa that I've seen before include "Old Roman admission of guilt" , "Acknowledgment of error or guilt" , "Acknowledgement of one's own fault (Latin)" , "I'm guilty (Latin)" , "Admission of error (Latin)" .)