The most common and most likely answer to this crossword clue is the 13 letter word UNDERACHIEVER
U
N
D
E
R
A
C
H
I
E
V
E
R
Buddy explains!
The straight part
We know that "One doing less well than expected," is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one doing less well than expected, = underachiever.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.