The most common and most likely answer to this crossword clue is the 7 letter word PLANNER
I've seen this crossword clue in The Daily Telegraph
P
L
A
N
N
E
R
Buddy explains!
The straight part
We know that "Designer/calendar" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case designer/calendar = planner.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.
What does PLANNER mean?
PLANNER noun- One who plans.
- A notebook or software in which one keeps reminders of items such as appointments, tasks, projects, and contacts.