The most common and most likely answer to this crossword clue is the 4 plus 4 plus 4 plus 2 plus 3 plus 6 letter word HAVE ONES FEET ON THE GROUND
I've seen this crossword clue in The Times
H
A
V
E
O
N
E
S
F
E
E
T
O
N
T
H
E
G
R
O
U
N
D
Buddy explains!
The straight part
We know that "to be realistic" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case to be realistic = have ones feet on the ground.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.