The most common and most likely answer to this crossword clue is the 7 plus 6 letter word MASTERS DEGREE
I've seen this crossword clue in The Times
M
A
S
T
E
R
S
D
E
G
R
E
E
Buddy explains!
The straight part
We know that "an MSc, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case an msc, say = masters degree.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.