The most common and most likely answer to this crossword clue is the 4 plus 6 plus 5 plus 2 plus 1 plus 3 letter word ROME WASN’T BUILT IN A DAY
I've seen this crossword clue in The Guardian
R
O
M
E
W
A
S
N
â
€
™
T
B
U
I
L
T
I
N
A
D
A
Y
Buddy explains!
The straight part
We know that "Be patient" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case be patient = rome wasn’t built in a 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.