The most common and most likely answer to this crossword clue is the 7 plus 8 letter word COMPLEX SENTENCE
I've seen this crossword clue in Financial Times
C
O
M
P
L
E
X
S
E
N
T
E
N
C
E
More clues leading to the result COMPLEX SENTENCE
Buddy explains!
The straight part
We know that "Punishment handed to Oedipus for patricide and for marrying his mother? Could be" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case punishment handed to oedipus for patricide and for marrying his mother? could be = complex sentence.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.