The most common and most likely answer to this crossword clue is the 4 plus 8 letter word HEIR APPARENT
I've seen this crossword clue in The Times
H
E
I
R
A
P
P
A
R
E
N
T
More clues leading to the result HEIR APPARENT
- Future leader’s great deal over Irish cause
- Pot broken by Irish relative, expected replacement
- One to succeed in declaring the invisible visible
- & 17d Airmen’s odds paper thin, sadly — one expected to succeed
- As eldest child, male, I severely criticise Dad
- Prepare a hint, cryptically, for person sure to succeed
Buddy explains!
The straight part
We know that "Future leader" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case future leader = heir apparent.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.