The most common and most likely answer to this crossword clue is the 2 plus 11 plus 3 letter word ST VALENTINE’S DAY
I've seen this crossword clue in Financial Times
S
T
V
A
L
E
N
T
I
N
E
â
€
™
S
D
A
Y
Buddy explains!
The straight part
We know that "With love, shall we mark it" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case with love, shall we mark it = st valentine’s day.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.